To be honest, I'm not that unhappy with the default look, but the designer in me keeps urging me to try and do something with my UI – and I always seem to fail to come up with better unit frames. Blizzard's frames have some features that I really don't want to lose, but I haven't been able to find something that does all this well:
Health/Mana text on mouseover
Threat glow and percentage
Backdrop colored by hostility/reputation/pvp-enabled, and tapped state
Dispellable/stealable buff filter for target frame
Level in relative color, and possibly in a separate frame
Portraits (duh)
All the frames I've tried so far has failed on one or two of the above, mostly when it comes to backdrop coloring and text on mouseover... I managed to come fairly close with Stuf, but not close enough. I know I can get some of that information on tooltips/via other addons but I would really like to have them in the unitframes if possible.
Easy configuration is a big plus (I'd like to avoid dogtags or scripting)... And of course, the frames have to be fully skinnable.
I've done a bunch of those before:
Target frame bar texts - this post (there's one further up in that thread where it's faded out unless moused over, I believe)
Highlighting of dispellable buffs on target - this post
The other stuff? Maybe. Currently have a bunch of other projects though.