This screenshot here shows pretty much everything you'd need to know as a shadow priest, buffs, debuffs on both you and your target in a nice minimal way: http://www.wowuigallery.com/bb-galle...510_141726.jpg
and again here: http://www.wowuigallery.com/bb-galle...710_114311.jpg
as a tank. I could see my threat on the target (the small red bar above my player frame), I am tracking my own buffs above my frame and I am able to see my debuffs on the target as well as any buffs they may gain for taunt rotations. Then the ToT frame shows the ToT debuff so I am able to taunt from them if they receive a debuff.
One improvement I did make to the above UI was placing my own debuffs under my player frame so I knew if I was standing in something or had a nasty debuff I was able to ask for a taunt if my co-tank was too slow.