Unfortunately, what you are asking for is practically impossible, since each class has different nuances. Tracking reagents, simplified spellcasting, stance switching, things like that are often accomplished by individual addons for each character. I have 3 addons just for my warlock (although 2 can be used for other caster classes), 3 addons for my warrior, and 1 additional addon for my druid (although I havn't played that one much).
If you just want to improve appearance (rather than gameplay), my personal choice for best "starter" UI is Nurfed. It provides a lot of the basics (better buttons, cleaner unit frames, smarter tooltips, etc), and is fairly simple to set up. The unit frames are still my favorite out of any UI (in fact, that's the only remaining part of it I still use).
Download the beta off their site:
http://www.nurfedui.net/download.php...rfeduibeta.zip
If you want to keep it clean, don't install nurfed_hud. It's mostly just eye candy.