Skeletal animation support! Quake3 doesn't have any bit of it, so you can't see really cool animation on characters because Quake3 simply doesn't support it. Try playing a mod such as Urban Terror and look at all the bodies that simply look like they are floating around instead of looking like they are running. If you put in skeletal animation in Quake3, mod makers can make players that look realistic when hit and that would add another level of depth to Quake3.
With the most recent patch and Team Arena, Quake3 is shaping up good for mod makers, but mod makers still can't really show off some sweet animations because of Quake3's limitations. Half-Life has this and its quite visible in mods (such as CS). Animations aren't perfect, but they blend together quite well to give an indication of what the models are doing.
vash