After all the insanity about invasion that was going on here when it dropped, I wanted to really get a taste of it before posting any thoughts. I'd just like to say, I really do like it, but it could become something unbelievable with a few tweaks. I'm going to focus on standard invasion here, because I've only played about 3 games of invasion slayer so far, so I'll reserve comments there.
Phase 1:
I'm going to go way against the grain here, and say that bungie made the right call with no Nerfle loadout. I can imagine a Nerfle Loadout just resulting in 5 of the elites staying back and trying to beat the spartans in a shootout. Nerfles on Map is a great way to balance this, but their location needs to be more apparent, so a few elites can pick them up, and give support to their battle bro from range. As is, I almost never see anyone picking them up.
Phase one is probably my favorite from both sides, as I really enjoying making that run under heavy fire into the base, watching my Battle Bro go down, and spawning him.
I also like the layout a lot, there are 4 stairs into the base underside, and 3 entrances to the top side for the elites. There's a lot of opportunity for misdirection.
Spawn camping needs to be addressed though, lots of good ideas on this forum already, so I won't add any.
Motivation for the spartans at this stage is weird though, sometimes I'll let the elites cap it because I'm having so much fun. and don't want it to end. :lol :lol I'm sure in the final version, with team switching it will be better.
Spawning
Spawning on your bro is one of the greatest mechanics in the history of mechanics, as long as your battle bro is someone you can communicate with. It makes you value life a lot more, it opens up new stragies (one BB be invis with a nerfle, give support and back out when things go bad) and it speeds up the game. It needs some tweaks though, especially more feedback about what is preventing your bro from spawning back in, so the BB knows which way to go. Seriously, don't ditch this system, just fix the glitches and feedback Bungie.
Default spawns for elites are wayyyyyy too far away though, especially in tier 3. Spartan's crazy spawn room is too close at the end.
Phase 2
Probably my favorite phase, good objectives, good layout (love the top window over the core cap point). The vehicles are good, without being too crazy here, and without a hojillion counters. Ghost, hog and wraith all fit, and feel really good, spartans have limited Vehicle counters, as do the elites, forcing some preservation of resources. Spawn camping can be an issue for elites again though, as they don't have a nifty one way shield to protect them, and the distance at this stage is just damn frustrating.
stage 3
I won't re-enumerate all the issues with vehicles in stage 3, but I have some slightly different solutions than other people. I don't think the Pro Pipe should be removed from the loadout list, but it should be really limited. Like, no armor ability, or only get two shots and no secondary limited.
EMP blasts should also be less effective, so if you get tagged it's not "ohh, I'm dead already." Maybe require two EMP blasts, one to slow the thing down, and the second stops it cold.
Spartan's shouldn't have an super easy, filled with weapons spawn to stop the elites at the end, instead how about some mongeese to close the gap quickly?
Sword/Shotgun spawning is fine by me. They are only useful until the elites get the core in the open.
conclusion
The more I play Invasion the more I like it, just tighten up the load outs and vehicles on tier 3, get the spawns working as intended, and give some more info on where the weapons are for each tier and it will be something really special.
I can't wait for generator defense, and more invasion in the future, it's a mode with a ton of potential. I really love the way spartans and elites are balanced against each other, and the way it feels to blast away at the other species during epic charges, in tiers 1/2 it really feels like I'm playing co-op campaign against human players. Focus on that element, turn down the "HOLY SHIT WE NEED MORE SPLOSIONS" at the end.