Like anifunk, I react faster to colour changes.
Instead of center icons, I use colours (Health Bar Colour) for debuffs. For poisons the frame turns bright green; for curses purple; and for a debuff that I can't remove but have to deal with, the frame turns really bright pink.
For each fight this is something different, but it's always the "UH OH" debuff that I'm expecting. On Bloodboil it was Fel Rage, on Illidan it was Dark Barrage, on Felmyst Encapsulate, etc. I don't have to think or interpret - as soon as I see someone's frame turn bright pink, I can switch to healing them. Or if they turn green or purple I can hit my decurse/depoison macros without really even thinking - it has become almost automatic when I see those colour triggers.
I also make use of the borders like Nitz - I have no need to show who I'm targetting. Instead I use borders for aggro (red border) and low mana (blue border). If someone's border turns red, I can throw them a rejuv and wait to see if I need to Swiftmend.
I use corner text for Rejuv & Regrowth, corner squares for buffs. I use a yellow corner for WG (I don't really care about the timer). I don't display other people's buffs, but I like the idea of using the border to show whether the target is Swiftmendable... might have to try that one.
This is my current setup:
I use GridStatusHots, GridStatusLifebloom, mana bars, and corner indicator text.