I don't play Nexuiz as often as I'd like, but I've kept track of each new version to see improvements. I've also advertised it to my friends, and have gotten feedback, both good and bad, about the game. I also read a lot of gaming news and talk with friends that like the hobby, so my thoughts hopefully reflect the average US shooter fan.
First, the maps on Nexuiz need a tuneup. I'm not saying you need to go wild with polys, but you need to stop making boxy maps. Unreal Tournament 2004, Halo 2, Gears of War, and pretty much any popular US shooter these days has cast aside simple maps in favor of rugged, uneven terrain and a plethora of odd shapes and details. One friend I was trying to get hooked on Quake-style shooters flat out refused to play them on the basis that the maps are too minimalistic and dull.
http://static.flickr.com/73/207711616_fbadac0d37_o.jpg See how indoor corridors have things like furniture and rubble?
Second, you need to modernize the gameplay. I love Quake-style gameplay, but it doesn't cut it anymore to the average gamer here. Things like cover, vehicles, team modes and exciting new weapons like Half Life 2's gravity gun are what interest players. Unreal Tournament's most popular mode was Onslaught, a team-based mode. Gears of War, Ghost Recon, and Rainbow Six thrive on both teamplay and rewarding smart players over fast shooters and big guns. In these games, run and gun is a deathwish, and a coordinated team can destroy a bunch of loners. Not to say you have to include vehicles or wild weapons, but they certainly are appreciated!
Last, I think a little bit of novelty helps. One friend really loved the idea of Nexuiz's Grappling hook. Maybe designing some maps and modes that force high flying? And honestly, one kickass mode or a new, kickass way of playing can really improve people's opinion of a game. My friends and I enjoy 1v1 Gears because the gameplay forces you to literally kill your opponent with him never knowing you were there. But don't just copy off of successes. Blaze your own trail!
Sorry for the mouthful. But in short, it seems like to get popular with American audiences, you need to drastically change the current nature of Nexuiz. What do you think?