Questions:
What don't you like about the UI? Be specific. What's confusing?
It feels like the game was made by two different teams. Out in the outer layers of the menu, you have the classic Halo style menu, which layers down into other options. Once you go into multiplayer, it suddenly switches to a awkwardly big tile based system. And the tiles don't make much sense. Player names are vertical but also on their side, so you have to tilt your head to read it. It feels like a decision made just so you could have spartan poses and have them visible. Then they went and layered them over each other in the lobby, so you STILL have to cycle through everyone to see their armor like Reach.
And the traffic flow (I don't know what other devs call it) is all wonky. When you press start and go down to the options, for some reason the least trafficed option (video settings) is the first one, and the File Browser is the last one, when I need to access my files much faster. In Reach, you were only two steps away from saving recent films. You have to go digging with a shovel to do this in Halo 4. Also, the file browser doesn't even work and you can't see recommendations if you have too many friends, so I can't remark on that because the feature is basically absent at the moment.
I'll look around later today and actually take notes, but the big word here is digging. It feels like you have to dig to get anything done in Halo 4's UI. Reach's UI was brutalist but it knew it had to give you access to your files in a straightforward manner. It feels like Halo 4 is trying to be pretty and not functional.
Which weapons are overpowered?
Haven't played enough to judge. Incineration cannon feels overly beefy. Counting vehicles, someone needs to realize Scorpions don't belong in regular matchmaking.
What are some big annoyances with maps or gametypes on certain maps? Are there parts of maps not utilized well?
There's tons of dead space on certain maps. Complex. Vortex (I think it was Vortex, anyway). In smaller games you see people hiding in corners and waiting for kills, especially in SWAT.
Gametypes: So many custom game options are broken or forced in the Megalo script I wonder why they bothered exposing them to us. In Reach, they said they didn't have the time to change the UI to expose more Megalo options to us. Then 4 is a new game, and those options are still only accessible to 343's scripters.
What's missing in the custom games menu? What can't be removed or edited?
see above
No precision editing, the objects shift around after saving about as badly as Halo 3, Magnets don't actually make certain objects meld together (there's a gap that you can shoot between/throw grenades through), the Impact space station pieces don't all perfectly match up if you're trying to make a map out of them.
The Forerunner lawnmower sound. The fact that there's hardcoded softkills in the map's playable space at all ( Ravine , Erosion ) or the fact that the building on Impact is not aligned on the grid or at an ortho angle (it's not at 90 or 45 degrees, I think Pete figured out to be something like 38). The map overwrite bug from Reach is still there. Pieces now force your Monitor to look at them from a distance, and you can no longer zoom. Pieces push themselves out of the ground (especially on Ravine) even when set to Phased. The Forge lights aren't included in the lightmaps when generated. Loadout cams are still there but pointless as they coat the entire screen in vaseline when the map starts. The fact that you have to wait about 15-20 seconds for a Forge session to actually end because someone though Forgers cared about the scoreboard at the end.
The Forerunner lawnmower sound, again. Also, pieces before that fit into even WUs (1.0, 2.0, 2.5 and such) are now weird sizes (3.2, 4.3, etc). So we have no precise editing, and the pieces are unevenly sized, so you have a hell of time even fixing all the z-fighting this creates.
Also, the same damn rocks from Forge World, including Rock Seastack, the most useless of Rocks. Tempest had the Spire, 3 piece that was MUCH better and more usable, and I don't understand why that piece didn't come back.