For me, the joy of DMing is more than 50% of the fun, it's around 80%. I love watching the players try and figure stuff out. A recent favorite moment for me was when the rogue fell into one to many pit traps and swore two things: 1. To get better at searching for traps & 2. To start looking at the celing. His reasoning was that, with so many pit traps, the builders had to design a few celing traps to balance things out. There were no celing traps, but I wasn't about to tell him that. :-D
Another one was a kid I have in my group. He dosen't role play very well, so I had a druid hire him (just him, not the party) to find out who was poisoning the ground water. The kid found the guy, killed him, and found 2 flasks, one with green liquid, and one with brown liquid. At first, he thought they were postions, treasure for the encounter. But, while I was working with another player (this was just after a big adventure and the players were roleplaying some downtime away from eachother) the kid realized what the flasks were for and "got it."
My point, and I do have one, is that these moments come from how the DM runs the game at the tabe, not how well he prepairs for session. Preperation helps, a lot, but it dosen't take the place of running the game. It is, first and formost, the DMs job to run the game. You can homebrew a world to rivial Middle Earth in scope, elegance, woder, and beauty, but it is all for naught if you railroad the PCs, don't breath any life into the NPCs, leave out imortant details the PCs should know, or any or the other sins a DM can commit.
As someone mentioned earlier, running a published adventure is more work at the tabe. It is an exchange, more work at the table for less work away.