Its hard to say what improved exactly. Game still kept like nexuiz and it still compatible with nexuiz and you still can connect to new servers with old 2009 nexuiz client. We make a lot of additions, single player campaign with quake and freedoom maps. we have our version of monsters. we add survival game mod, a lot of in-game improvements and additions. You can check rexuiz github commits list to get step by step what was fixed/improved/added.
Well, project was started as mod to nexuiz like RM or like a replace of RM mod. But we found what default nexuiz client from 2009 cant supply support to all our additions, so we update game engine to actual changes and rework almost all nexuiz game code. The difficult thing it a back compatibility. We tried back compatibility for xonotic also, but xon devs banned all servers with such experiment on their serverlist. So we left back compatibility for nexuiz only mostly to inform nexuiz players what there is a new version of game and slightly forcing them to update :)