I DM 3.5 for about 4 years now (shorter campaigns, lasting from one-shot to half a year) and after some serious reconsideration and some documentaries (Dungeon Masters (2008)) I am realizing that I like the most to tweak encounters (in broadest sense of monsters, traps, puzzles and RP shenanigans) to a thin point between beateable and unbeatable for my players. Its a bit like tunning a graphic card. How far can I push it before I ruin it? Also, I live to see people overcome my rather impressive obstacles.
I admit, there were a few ruined cases of TPKs and such, but people still love to have me as a DM.
Also, I love to use powers, abilities and SLA monsters have instead of just stacking hp, AC, to-hit and dmg. A lean monster with low AC, low hp, yet maybe some decent defensive abilities and silly high init. mod. and party-wrecking abilities? Awesome.