My impressions:
I'll skip the part where First Look, Microsoft and 343 managed to create a horrible setup where probably 75% of the people coming for Halo 4 didn't even get to play the actual game. I see Robbert already covered that.
I'll get to go good stuff, but just in bullet points because fuck you that's why. (edit: Okay, it ended up becoming quite a story)
-I played Regicide on Longbow twice with a total of 8 players.
-To my surprise there wasn't any input lag.
-I won my first game with 295 points with 27 kills and 7 deaths.
-The second game I came in second with 286 points, 20 kills and 4 deaths (more on this game later)
-I really dislike Regicide because while it definitely improves some aspects of Juggernaut, it doesn't solve all the issues. The main reason for my distaste is because you essentially get punished for doing well. If you're the King in Regicide you have a waypoint above you head but that's not really the biggest issue, the biggest issue is that fact that you can't score nearly as many points as people who aren't in the lead. It encourages people to play (relatively) bad up until right at the end. I'm not entirely sure how the points are calculated for killing the King but the highest I saw for killing on was 50 points. In my second game I was in the lead nearly the entire time and near the end I had 285 points while the guy in second place had roughly 250 to 260 points, I can't quite remember. I was fairly sure I would win but then after getting killed once the game is suddenly over and I'm in second place. Apparently he happened to finish me off at the end while I was a one-shot and he got 40-50 points for that single kill. Afterwards I checked the stats and found out that he ended up with 11 kills and 8 deaths while I was in second place with 20 kills and only 4 deaths. The guy was definitely a worse player than me but I felt like I lost because of the scoring system and not because I didn't play well.
-Giving the King an overshield after a certain amount of time (Which isn't communicated) didn't really do it for me. At one point I was the King and I was in the middle of a firefight with someone. I took some cover for a short time because I was losing the battle and all of a sudden I got an overshield for being the King. Then I quickly got out of a cover and easily got the kill. It felt really cheap.
-Can't really say much about Longbow because it's clearly not meant for 8 player FFA. Sprint helped a bit but most of the matches I spend half the time looking for people.
-A random thing I noticed is that the motion tracker shows what direction vehicles are coming from even if they're outside the range of the tracker. There's an arrow on the edge of the tracker pointing towards where the vehicle is.
-Another thing that I noticed was that there was a glitch where your reticule wouldn't appear until after a about half a second after you zoomed in with the scope. This is a bit worrying since I'm pretty sure we were playing the latest event specific build of the game.
-Having sprint on A and the AA on X was really annoying in the short time that I played. I will get used to it but I would've made more sense to just keep sprint on X.
-I didn't pay close attention to the movement speed or agility of the Spartans so I can't really tell too much about that.
-The ordnance weapon drop system seemed terrible. At one point during a match I was on top of the bases about halfway through a match and suddenly there was a bright flash and I heard an explosion. I jumped down from the roof only to find out that a power weapon dropped right where I was a second ago at the top of the base, right on top of me. There wasn't anything warning me that a power weapon would get dropped on that location at that time. It just dropped from the air and I could only see it the moment it hit the ground and it showed up on the radar and it had a waypoint above it. This worries me a lot because it can fuck over teams that are in the lead but just not in the right random position at the right time. I still don't understand why the system is in the game in the first place. Just having waypoint above power weapons would've been enough. I really don't buy the argument that it was implemented to prevent weapon spawn camping. This only really happens at power weapons placed on maps that shouldn't even have been on the map in the first place like the Sniper on Hemorrhage anyway.
-I only checked out a handful of weapons. The Battle Rifle, Light Rifle, Carbine, Sniper and Rocket launcher were all the weapons that I really used. I don't even think I ever saw any other power weapons aside from the Sniper, Rocket Launcher and Spartan Laser anyway.
Of the primary weapons that I used the Battle Rifle seemed the most useful. It was very effective at short to medium range and was fairly useful at long range.
The Light Rifle is in my opinion not that good at short to medium range because it has a pretty slow rate of fire. It's definitely better at long ranger though when you're zoomed in.
The Carbine is poop all around. Recoil seemed crazy on the thing, probably more useful on smaller maps because the rate of fire is high.
I only used the Sniper Rifle for a short period of time and it felt like something in between the Halo 3 and Reach Sniper Rifle. I imagine it being close to 3's on xbox live though.
The Rocket Launcher seemed pretty powerful, not as powerful as Reach's though. No complaints except that the trail behind the rockets looks dumb as hell.
-Of all the AAs I only used Promethean Vision and the Thruster. Well, honestly I only used to the Thruster once and that was on accident. I rarely used the AAs anyway but that may partly be the cause of me never using anything besides Sprint in Reach anyway (Unless we're playing 2forts).
Promethean Visions was really useful. You can see people practically across the map within about a second after using it. People saying you won't need it because you have a radar are crazy. The range of PV extends way beyond the border of the motion tracker.
Can't say much about the Thruster. Looks like it's disorienting more than anything, especially since it's now in 3rd person.
Overall I didn't see many people use any AAs really, but I think we'll see that change once people get the hang of them.
-The first perks I went with were Dexterity and Shield. Most of the other ones seemed useless to me and the two I chose could come in handy a lot more often than the others.
Having a faster reload time was incredibly useful, especially if you had a power weapon like the rocket launcher. Seriously, that shit reloads fast now. It's a bit silly if you ask me.
Having a faster regenerating shield felt like a must to me. When I didn't use it the shield recharge time felt too long in my opinion.
I also used the Firepower perk but that was mostly to checkout different primary weapons without having to switch loadouts. It's also helpful for when you run out of ammo for your first primary, doubt that will happen too often in Team games though. And because of the size of the map with only 8 players on it I didn't put up weapons from enemies as much as I could've.
-The audio sounded like everything was a lot more... metallic. This is - of course - especially the case for the UNSC weapons but the Carbine also sounded like it was shooting rounds rather than plasma. The Covenant weapons lost a bit of their character in that regard. Not that I particularly care, but whatever. The shield recharge sound also sounded different. I wasn't the 'whoooom' sound anymore but more of a 'tsss - clunk' sound if that makes sense. IMPORTANT STUFF
-I'm still not a fan of the menu. Having to flick through all the 'cards' to easily read players' names seems like a step backwards in that regard. I also noticed that you can't even see someone's servicetag unless you highlight their card. Another reason why I dislike the card system is because it wastes space that could've been filled with the active roster. I quickly checked the roster out and it looked like you could see less players in the list and you had to bring up a menu to see who's online. Coming from Reach's slick UI where everything is right there in front of you made it a bit cumbersome. I don't want to go into a different menu to check who's online, the fact that you could see who's online all the time was what made it awesome. Overall the menus seem to waste a lot of space by having huge squares with a single option on them, a shame.
In the end I couldn't get a really good feel of the game mainly because the gametype we played along with the combination of the map was horrid. Shooting feels nice but on LAN it pretty much always is, even Halo 3. AAs and perks didn't seem to impact the game a lot, it was definitely more of a gun game like others have mentioned. But this could be because people aren't used to them yet and are still experimenting or because it was a casual environment in which I played. In more serious matches I can see them becoming frustrating at times. I doubt you will feel cheated out of kills or wins very often because of AAs and perks but I'm positive it will happen often enough for it to become annoying.
I am worried about the direction that 343 is taking with Halo 4 because it keeps adding elements that have caused me to stop playing a certain other franchise. I stopped playing Call of Duty because too often I felt like I was cheated by the game. I often felt like I didn't die or lose a game because I played badly, but because the game mechanics screwed me over. In CoD you will never know what to expect from your opponents, the game becomes very unpredictable and not in a good way. 343 can use the word "dynamic" all they want but I don't always see that as a good thing. Halo was awesome because it was simple and you it was predictable. Not in a way that you predict what will happen during a game - enough crazy 'random' stuff happens in Halo - but the way of knowing what you're up against. You fight against 4 other dudes with a BR primary, AR secondary and 2 grenades. On the map are X, Y and Z power weapons in locations A, B and C. That's it, you know what you're up against and skill will be the decisive factor. In CoD this isn't the case at all and you'll have to guess what kind of player you will be facing. Will he have extra armor? Extra ammo? Faster movement speed? 3 grenades instead of two? You don't know. Obviously there's still a lot of skill involved in Cod but in a different way than Halo. This is what people mean when they say that CoD is more rock-paper-scissors more than anything. They don't mean literally having a few things that counter each other perfectly, they mean that you can't predict what features the other player has so it becomes a bit of a gamble. Some people like this but I'm not a fan of this at all.
I'll be clear about this: Halo 4 isn't even close to being that bad in that regard as CoD but it's definitely moving into that direction and that's probably what's most worrying. Because that is not Halo to me.