Your character has abilities, useful ones even, that you just forget to use.
I’m still not used to using the Bag of Tricks I have on my Vulpera. I probably don’t go berserk enough on my Troll Hunter. And I know I don’t use the long cooldowns on this character as often as I should.
What else do you wind up not using like you probably should?
I have a lot of trouble remembering to make use of my sigils, tbh. Its just not that important in my rotation I guess, except the damage one
I sometimes forget about my portals
Hibernate can be useful in some situations, but I always fail to remember I have it let alone use it.
I often forget Imprison. Mostly because there’s very little need to CC anything on a regular basis.
I usually forget about the racial abilities and some others like long CD ones and utilities. Especially so on less played characters.
War Stomp. I’m so used to not using it on my Tauren Hunter that when playing my Paladin, DK, or Monk I forget that it is a helpful ability for them.
Been playing a monk for four expansions now and I still never use Transcendence. I know it could be handy in the right situations and I even have it keybinded. But I really never use it and often forget I have it. >.>
Disengage. I never use it even when I should. I just never remember it. Though I wish it had a glyph or a trainer option like the mages blink that lets you disengage in the direction you are moving instead of just straight back. It would make it a hundred times more useful and I might remember it more.
Definitely Eye of Kilrogg. That’s a good scouting tool to use before going deeper into a dungeon, or into an area that is partially obscured.
Aside from shadowmeld I always forget to use my racials. My undead deathknight will be feared/charmed and I’ll completely forget about will of the forsaken.
I also forget to use a lot of the less common utility abilities on most of my characters. Earthbind totem/tremor totem come to mind, soothe/hibernate, control undead, etc.
I forget I can bubble on my Paladin quite often.
In blood spec I always, always forget I have a friggin 30 yard no cd (except rune requirements) long range pull ability. I don’t even remember what it’s called right now.
On the rare occasion I remember and I’m tanking, it’s great fun to just fire off 6 bolts and pull everything. But this usually never happens because I completely forget it exists!
Recently started playing a Mage and forget about my barrier all the time.
I forget I have flares. 
lolwut?! Silence is the most important one, followed by chains. how do you even play without them!?
Imprison also works as a second interrupt. Slowly I realize why DH players are so bad.
Well I’m a tank. Unless I’m in PvP, I can pretty much just sit there and wear them down with the usual DPS and demon spikes - consume magic if there’s a caster. I rarely run into situations that my casual-self needs a fear or mass-silence
Unnecessarily rude
Only against certain enemy types, among which the predominant enemy type of this tier, aberration, is not included. There are very few situations in which I require a second interrupt that don’t involve a group, in which case there are other players with an interrupt available.
And as long as we’re quibbling over stupid crap, it doesn’t double as an interrupt. Interrupts cause spell locks. At best, it’s no different than a momentary LoS, which won’t last even a full second.
I just hit 60 on the vulpera I’m leveling, and realized I had’nt used my heirloom tentacle trinket… at all.