I admit I was sort of "meh" about getting a copy of Diablo 3 with my year's subscription to Warcraft. Warcraft does suit all my needs, and I'm not looking for other games. But friends were playing it and so I decided to eventually try it out because it's a solo game and sometimes I want to fiddle with something while I'm writing something else.
But once I got into the game a little bit, I discovered that a) most of the info out there was the result of rewrites of the pre-release game (and two years old) and b) contained in threads in a discussion forum. So you have to slog through page after page after page (after page) of comments just to find out what stats you should put on the Rogue who's following you around.
So -- I haven't played it up to the highest level, but here's the basics (this is just MY opinion. Your mileage may vary):
1) It's mostly a solo game. You can join the game with friends or join a public game (if you're high enough) but I've played it mostly solo and I'm going to assume most folks here will play it solo as well.
2) You'll reach the very last boss and the end of the game about the time you hit level 30-34. Don't worry. This means you restart the game on the "it's harder now" level, where you really do need to be at least level 30 to beat up all the monsters in the starting area.
3) The best choices for "I never played Diablo before but I want to try Diablo 3 just for grins" are either the Demon Hunter (if you like to stand far away from monsters and shoot them to death) or Barbarian (if you don't mind smacking things to death. Each class has its plusses and minuses, and this is just MY opinion.
4) You will get gold. Lots and lots and lots of gold. The merchants in town will want to sell you stuff, but it's going to cost you more than you can afford. Your first purchases should be health potions because everything around is going to want to kick your tail from here to next Wednesday.
5) Dying is not permanent unless you're playing "hardcore." Play through the game once before you go "hardcore" in Diablo 3. And when you die, don't forge to go back to town and repair your gear.
6) Go out of your way to kill everything... and I mean *everything*! Your first character will be pathetially geared, so in order to survive you need gold and gear.
7) Make a secondary character. There's going to be stuff you want to store or keep for later. You have a chest (bank) that appears at all your main bases. It costs gold to buy additional slots. It's much easier to create another character (with lots of bag space) to hold all those goodies. You can play the character later if you like. Just create them and run them into town (kill the first few monsters, do the first quest to open the gate, and sit around town waiting to stuff things into their bag.)
8) If you're getting killed lots, then exit to the main screen and start the previous adventure again. In "hardcore mode", I had my character run down the hill and kill things again and again until I hit level 2. Then I talked to the Captain, killed the incoming zombies, and got inside the gates.
STARTING GEAR FOR FIRST-TIME PLAYERS:
For your first levels of kills, equip ANYTHING that fits! Even gray colored (worthless) gear. If you hold your mouse over the item, a box will pop up telling you about any improvements if you equip it. If it's got better stats (or if you don't have one yet (like bracers)), equip it.
To delay completing a quest, just sign out of the game and then sign back in and you'll be at the starting point for that section of the game. That'll make the area monsters easier to deal with.
You can go visit the Auction house and gape at the prices (there are two sides to the auction house -- one where you buy stuff for in-game gold and one where you buy it for real money. I am a chicken. I haven't spent any real dollars and cents on items nor have I tried to sell them for hard currency. I'm sticking with the in-game gold, thanks.
At different levels, different vendors will appear. The first one is the miner and the bartender inside the inn (where you can buy potions.)
There's achievements for things like talking to all your companions (a blue-white asterisk will appear over their heads when they want to tell you more of their life story. You can turn the sound off and read their comments (I usually do.) There's an achievement for buying dyes and dyeing all your armor (fashionistas in Diablo -- who knew?)