What you think is impressive may be crap to someone else, just as what they think is impressive may be crap to you. For me, impressive is my Yogg-Saron kill, as it was our server second, alliance first kill. We also got the "Nobody goes insane" achievment (forgot the name). Of course, to any hardcores, Yogg-Saron normal mode is cakewalk. They think of Algalon as hard, and I haven't even done him.
TL;DR
Impressive is subject to the interpretation of the viewer.