Over the last week I've been playing a lot of Diablo 2. I've got past its datedness and been sucked right in by its fascinating range of possibilities and addictive loot generation system. Of course it helps that it's possibly quite a good way to prepare for Diablo 3.
So what have I learned playing Diablo 2 that might be relevant?
First, melee is harder than ranged. While we can't quite be sure that Diablo 3 will play similarly to its predecessors that's a view which is shared by some of those who've tried D3 in Beta. Initially in D2 the person I played with choose a melee paladin with a two-handed sword - he died constantly in Tal Rasha's Tomb at the end of Act 2 Normal. So we changed it to a trapper and a zookeeper and that seems like a combination that will do well in Diablo 3.
Both classes have off-screen ranged power. To get maximum range in Diablo 2 you need to set the display to 800 * 600; fire a spell that lands in a corner of the screen and which damages mobs off-camera. He uses traps to do this I use bait golem. Chuck a golem in and if nothing wakes up and attacks it, that spot is safe so move there and repeat. There are plenty of offscreen options in D3 (including summonses and traps).
One of the interesting differences between the games is the ease of respeccing. I've realised playing the modern version of Diablo 2 how fundamentally being able to respec has altered the game. Diablo 3 goes even further. Want to jump over that river? Swap in Leap Attack or Teleport and off you go.
This means that the "here's my build!" threads that I and others have made are slightly skewed in concept. You need several builds and should be prepared to alter them on the fly. Rather than being a "Whirlwind Barb" or a "zookeeper" a master Diablo 3 player will be able to use any skill situationally and will know the synergies.
Synergies brings me on to another fundamental difference. In D2 most people constructed their characters in a void. The game is based on a everyone is dps paradigm anyway plus there were few social skills and they were weak (eg Holy Bolt).
In D3 synergies between classes can be very powerful. Combined with the ease of respeccing you should be ready to swap skills in to suit other people. That's fairly explicit for the Monk where people are obviously going to ask for particular mantras. I also think many people coming from WoW is going to expect a Barbarian player to tank for them.
Here's my impression of what people will generally want from other players:
Barbarian - tank. My guess is most people who play Barbs will consider the class dps and most people who don't play Barbs will consider them tanks. People may ask you to use the shout that makes mobs drop extra treasure.
Demon Hunter - dps, snares
Monk - healer, buff. Expect to hear the most abuse if you join random games and don't heal people or buff them with the right mantra.
Witch Doctor - tank. People will expect you to keep up summons for them to hide behind (possibly while simultaneously complaining about how laggy you are to play with and how over-powered you are).
Wizard - dps
As ever playing within a more understanding community will insulate one from the horrors of the internet public.