It'd be helpful, in my opinion, if buffs and debuffs were displayed a bit more clearly. As it is now, I still find it a bit muddled and confusing.
TL;DR: consider the solutions below to make buffs & debuffs clearer to (inexperienced) players.
A few examples...
Debuffs applied by players. Think of groups with more than one of the same class, or multiple party members applying the same debuff, and (to a lesser extent) world boss runs. If there are multiple stacks of, say, Rupture on a mob, how do I know which is mine and when to reapply?
Debuffs applied by mobs while playing as healer. Tank almost died on Kongar a couple of weeks ago 'cause the healers weren't aware of the debuff. I was one of the healers, and I've done WB's plenty of times. Until then I had no need to know about the debuff, but when it was applied, it was lost among the tank's many other buffs and debuffs.
Buffs on mobs. Specifically boss' Enrage--this was/is not a new concept to me, however the first few times I experienced it in this game went as follows: more experienced player says boss is enraged; I mouse-over all unfamiliar buff/debuff icons to find out what Enrage icon looks like; icons bounce around as buff/debuff timers run out and new ones are applied, making it unnecessarily difficult to read any of their tool-tips.
A few possible solutions...
Display the icons of any DoT's/debuffs a player applies first. When I'm looking at the rows of debuffs on a mob, make it so mine are the first ones listed (top row, left side). Other players of course would see the debuffs they've applied listed first.
Make those icons appear bigger to the player so they stand out more.
Display buffs and debuff icons in completely separate rows. For example, if I'm healing and targeting the tank and s/he's got a row-and-a-half of buffs, drafts, elixirs, etc., any debuffs should start a new row underneath and separate from all of that.
Maybe put a red outline around debuff icons so they stand out more.
All of this just comes from being unfamiliar with the fights and icons, and may be unimportant or non-issues to seasoned players. But I do think the changes would help new players become more familiar with these mechanics and status effects.