MY first "Wow" moment would be getting my very first level 50, Spines/Regen Scrapper. I hit 50 in the last room of the Lady Grey Task force JUST before the Nemesis cut scene triggered.
Firing off the Electric Judgement power in the middle of three spawns before they fixed it twice over. When it went live the damned thing could actually hit enemies several times if it jumped around long enough. Secondly somebody forgot to remove the damage buffing code from the psuedopet. As on Infernal Infested, my SS/Fire/Mu Brute, with her three stacks of rage and continuous burn of reds, that was pretty ridiculous pretty quickly.
Also with Infernal, another fond memory was the first few weeks/months of AE, when you could fill a mission full of bosses. Being able to fight a churning sea of Freakshow Tanks and not merely survive, but THRIVE was totally awesome.
Taking down giant monsters with a low number of players was also amusing. My group had run a TF that ends in Dark Astoria, and after it was done we had four players left and Adamastor was up, so we figured "Why not?" Then later one I remember trio-ing ToT Eochai when me and a few others were taking advantage of Halloween's never ending night to farm BP Masks.
And then once Incarnates hit, I loaded up my inspiration tray and prepped using the auction house "use anywhere" command in order to take on a few GM's on my own. I managed to take out the Kraken, Adamastor, and Scrapyard with Infernal.
When running Incarnate trials it would always make me smile when I used Knock-Out Blow (again with Infernal), and could tell when the blow landed, because I'd see the UI show the health bar dipping by a good 2 percent.