And then it happened: This year, after 20+ years in the hobby, I started actually GMing.
I've always thought I could be a pretty good GM; I tried a few one-shots in the past with varying results (some good, some bad). I had to learn not to railroad, and it took a little bit of work to find the balance between preparation and flexibility, but I've always had a knack with creating interesting characters and story (I'm a professional writer by trade, and have done college-level and semi-professional theater), and right now my current Pathfinder group seems to be having a very good-to-great time.
But—It's become totally, brutally apparent to me now, having GM'd for six months, just how "heavy" the Pathfinder / 3.x rules really are.
The difference between being a player and being a DM and how the rules present to you is really significant. Most players I know enjoy options - having more and more things that their characters can do. However, they're only dealing with one PC. If the NPCs the DM uses have the same rules, then you quickly move towards overload if the NPCs are built on a level that requires a separate player to run each one!
I can't move back to 1e or B/X D&D because my players prefer the later editions for their character options. I've just found it's easier for me to run 4E than 3E.
Cheers!