I would surmise it's experience. I could put together a level 1 4e PC in under a few minutes or so. Pathfinder/3e takes me much longer now. I made a level 1 sorcerer for Carrion Crown and it took me a few hours to go through the various bloodlines, the traits, etc. that I wasn't familiar with.
I think I've settled into my head what the key difference between the systems are. 4e promotes balance as one of it's key pillars in design. A fighter is never made obsolete by the mage or cleric. Spellcasters are now more or less in line with martial classes with respect to their effectiveness. 3e had the traditional, fighters are strong to start but gradually see their effectiveness diminish (especially when you had the mexican standoff between two high level fighters waiting for the other one to get in close so he could make a full attack) where mages start very weak and minimal spell options. As they level their power level increases exponentially.
Personally, I prefer a slow and steady power increase to the one that starts off really low and ends really high. Again, not right or wrong, but that's a key difference.
Another major difference is on the combat side. 4e promotes the more epic, set-piece battles (which consequently take longer to resolve) as their bread and butter, which can be annoying as sometimes you just want to ambush that scouting party of kobolds, but they each take 3-4 hits to drop. (you can use minions of course). 3e tends to have smaller combat locations, less number of foes and combat tends to be a bit swingier, especially with save or die spells at higher levels and small amounts of hit points at lower levels.
For me I prefer the longer, more tactically focused battles, but some prefer the shorter, quicker battles. I don't think either's better, just different. I think 4e requires the DM to only put in combat's that have meaning to the story because you can easily get bogged down in somewhat meaningless combat encounters and leads to boredom. That's why most of the 4e modules have sucked large so far, they're just a string of combat encounters pasted together.