Best way to let you know a buff is active is putting in a simple UI option people can toggle if they so choose that overlays the time remaining of the buff on the skill on your skill bar. It's easy to do, the info is already there in the game, they'd only be adding a display of it over the skill. Frankly there's entirely too much on screen animations and effects to be able to effectively use visual cues for everything. Personally I'd prefer they open up the API, and let add-on authors do it, with my personal preference being an audio option of a woman with a foreign accent (German or maybe Russian) telling me a specific buff is down.

Just my opinion of course. Though seriously, just throwing up an optional timer over the skill would do wonders. Buff/Debuff tracking either needs to be remarkably more obvious in the game design tells (a very large amount of work, but doable), or they just need to lean on the UI for it (easy, and rather lazy, fix).