The biggest difference for me is the ease of DMing. The sheer joy of it again. Something I had not felt since 1st edition.
After trudging through the first series of Pathfinder modules it all crystalized for me. DMing
3e SUCKS. Maybe it doesn't suck for you, but for me it was torturous.
Even in a prepared module like Pathfinder I had to do a ton of prep. Every monster or NPC with a spellcasting ability had to be looked up, the spell details noted, and the ability modifier and bonus changes for buff spells all pre-calculated so that when the PCs encountered that monster or NPC, I knew all the bonuses they should have. Paizo tried doing some of the work for me, but I consistently found errors in their stat blocks. Or they would forget a spell that I thought was important. And god forbid the PCs dispelled something. What a nightmare.
Another thing that drove me nuts was them including a monster in an encounter and then instead of reprinting the stat block, they simply referenced a page number of where that monster had previously appeared. So now I had to have multiple pages marked with post-its so I could go back. Or they simply referred to stuff in the monster manual, so now I had to have multiple books opened in front of me. The adventure, the
MM, and the PHB (so I could look at spells). I had to have a little side table next to me to hold everything. Such a mess.
When I DM an adventure, I should be able to run the adventure solely from the adventure itself. During play, I should NEVER have to look up a monster ability in another book. Everything I need should be self-contained in the stat block and that stat block should appear right there in the adventure in the write up for that encounter.
4e got this right.
3e was an EPIC fail.
When
4e finally came out it was like the heavens opened and angels descended. The monster stat blocks were entirely self-contained. No need to ever look anything up. The first time I ran a
4e encounter it was like a dream. I hadn't had this feeling since 1e. I felt liberated from the accounting minutiae and drudgery of
3e.
Now
4e isn't perfect. I have little quibbles here and there. For example, I think there should be a mid-combat recharge mechanic for some powers. I think some rituals should be castable in combat. But on the whole its a vast improvement.