My opinion:
Always Online: doesn't matter, I would be playing it on battle.net anyway, so I can jump in with some friends whenever I want. That's exactly what I just did, playing with some friends with a character I spent some quality time playing solo earlier.
Auction House: Using the gold auction house (the RM one isn't up, I don't really care for it so it's irrelevant for me), I sold some items for some extra bucks so I can throw it in the gold sinks. Hey, money is relevant in this game, what a change from Diablo II!
Customization: I'd take D3 free-swap anytime to Diablo 2's "hey, play the first 20 levels without spending a single skill point, so you can dump it all on this single skill" mechanic. I've spent quite a while trying different skills, looking for a new gameplay for my wizard. Much better than trying to guess what skills does in D2, then wasting skill points in useless skills.
When I played Torchlight 1 I said "this is boring" to half the skill tree. Obviously I didn't spend a single point in these "obvious boring" skills, so I basically had almost no skills.
In Diablo 3 I said "this is boring" to a half a bunch of skills/runes, but I tried then anyway. A surprising number of then were actually really fun and now are my skill of choice. Take that!
Inventory: you're actually intended to ignore the white items after the beginning. Also, your inventory is big enough you can pick all blue/better items for a while before it fills up. In fact, the game flow is done in such a way that your quest will bring you to town before your inventory fills up, giving you a chance to dump it however you fells.
Also, if your inventory DOES fill up when on the run, you could use this as an opportunity for a quick break, use your town portal, go to town and clear the bag, check if any item is a good upgrade, repair, talk to NPCs for idle chatter and check if any of your new skills are interesting.
Length: it took me 16 hours to beat normal. Some idling, solo play, multiplayer with friends and with random people.
Gameplay: indeed, it's more of the diablo gameplay, but with new places, different skills and classes and fun. Hey, did I tell you it's FUN? That thing you play games for?