Some of these will be repeats for you.
Combat Roll + Pommel Strike + Opportunist -- Never got around to it until many promotions in, and developed a highly manuever-able style.
War Horn (upgraded) -- Reaver Expert Kaaami filled me in on its use in a public game one day and explained how awesome it was, how it synergizes with lots of things.
TTD -- Mixed feelings. Great for boss fights, but boss fights are short & infrequent outside of special events right now.
Fallback Plan -- it was meh to begin with, but it is highly skill dependent. After learning situational awareness with map awareness, can now win and troll enemies instead of be trolled. Note: don't upgrade this skill.
Blessed Blades: Sure it was free, but it was disregarded. With SWAC, attack speed has increased greatly making BB so much more valuable.
Leaping Shot: I wasn't a believer at first, but I could tell Full draw sucked, but Archers Lance could be awesome when I first used Archers with decent gear. LS however took longer to master, but now I do lots of trick shots with LS, backflipping toward or away from enemies as I please. Hawkeye and Arrow have nothing on this archer!
Sleeping Powder + Knockout Bomb + Mercy Killing: The sleep skills and guaranteed crit didn't seem all that useful until later discovering panick from firewall, war horn, and the shock + weakness aoe sleep abilities, and that incapacitates increased overall exp. Also discovery of sleeping RTC.
Mana Surge + Static Charge: Disregarded these passive until seeing it in action, now use them in every build, and being central to certain builds on both Keeper and AW, while still awesome on Ele too.
Static Cage + Disruption field. Had to be tipped off to these, was tipped off early and loved them ever since. Learned to keep DF on level 1 unupgraded for added duration, while wearing ring for added DF power.
Veilstrike: It didn't look like much in the beginning, but combined with static cage meant AoE Knockdown and Sleep. The improved weaken is amazing too.
Firewall + Fire Mine: Not dismissed, but overlooked until I saw another ele use it during perilous (month one when no one was running public perilous except these brave souls). Now a staple and amazing, especially with Ring of Fire Wall +duration.
Stone fist: On AW it was overlooked. Going no fade step or spirit blade was a revelation as well. The whole Combo king build depended on many advanced concepts and is a bit of a crowning achievement for me. No one else discovered, publicly played or announced that build and playstyle at the time, and it was radical. People still argue against it, but those that learn... amazing results.