Thoughts on Diablo 3 after having played for about a month:
1) Diablo 3 in normal difficulty is so easy you find you'd rather just skip it and play nightmare. Unfortunately, the game requires you to play through in normal difficulty anyway. Kinda takes away any feeling of accomplishment you get from progressing the storyline.
2) Servers are down, on average, about three hours a day. For a one-player game that requires you to be online, this is pretty awful.
3) Lag deaths in single-player game. Enough said.
4) There's virtually zero incentive for anyone to play multiplayer: monsters all have a lot more hitpoints so they generally mitigate any benefit from having additional players shooting at them. Plus, every player in-game has to be actively shooting at the monsters, or you're trying to kill a monster with three times its usual hitpoints by yourself. Oh, and if a new player joins your public game in the middle of a boss fight? Good luck.
5) The rate of mudflation is ridiculous. About three weeks ago, internet vendors would sell 1million gold for $4. Two weeks ago, $2.60. Two days ago, $1.30. Gold is being added to the system at a rate that an average player could not possibly keep up with by trying to farm on his own.
Having finally beat Act IV Inferno (took about a month, so that was nice), I find very little reason to continue to play the game. However, I haven't been involved in the PVP game yet, I might give that a go at some point. In general, though, I find D3 fairly disappointing. SoD seems to have way more long-term playability.
EDIT: 6) Oh yeah, and the cost of repairing your items when you die in inferno difficulty is prohibitive for the average player. You're either forced to buy gold from some internet vendor, or farm gold for a few hours in order to be able to afford progression. No thanks.