First off, tremulous is freaking awesome. It really is awesome to see an open source game that is original in content and gameplay, but simultaneously really addicting and polished.

i have already wasted way too much time playing (in game name either tuthpaiste or werepants). i hope that tremulous just gets bigger and better from here.
a few things i noticed: it is really hard to get a sense of when you are attacking as an alien. i know weapon models are in the works, that will help a lot.

it would also be good if aliens left some kind of "bullet-hole" on walls to help users see when they are attacking, and see what range is like for different aliens.
another thing: the radar needs some work for the aliens. my main complaint is this: it is kind of impossible to have it be usable at all when you are wallwalking, because it keeps working as though you were on the floor, whether you are on the ceiling or wall or whatever. so it would be good if it was somehow relative to your position, or more of a 3d radar like in a flight simulator.
with wallwalking, it is really chaotic, but that is part of the fun. it might be nice to have an inclinometer or something to show which direction is down.
it would be great to have a few more taunts or calls. it is really nice having the current sounds, but it would be cool to have a "help" sound, an attack sound, a taunt sound, or whatever.
some things to allow better organization and teamwork would be great. guild wars has a minimap that lets every team member draw and mark, but it fades off pretty quickly, so people can communicate there. that would be one nice option. another would be setting a "stink trail" or some kind of marker as aliens, and setting a beacon as a human. this could just be nice to show base relocation sites, a rally point, or whatever. it could really lend to the teamwork side of the game.
last thing, i really like the music on the intro, but it is annoying that it restarts when i click play, and when i go back, and all of that. it would be nice to have an options button on the main screen, rather than having to get in game to change controls and stuff. lastly, i would like an option to quit from a game to the server list rather than the front page, just because it seems like a more natural navigation step.
anyways, those are the few things i have thought of that could improve the game, but overall, i love the balance, the feel, the gameplay, and pretty much everything. it is a great blend between scary and crazy fun; kind of like quake mixed with team fortress mixed with aliens vs predator. keep up the good work!
