The attribute gain is 5 points per level, and they are mostly distributed in favor of your main attribute.
For example: Monk will gain at 2-3 dexterity, and the remaining points are distributed to the other stats (all stats won't necessarily increase, either).
The remaining stats may get a 1-2 point upgrade; I've been noticing a pattern where as a monk, I usually get 2-3 dex, 1-2 vitality, 0-1 int, 0-1 str.
So it would seem there's the main preferred stat, a secondary stat, and leftovers. :P
Quote from Game Director of D3, Jay Wilson:
Stat progression as a system is very difficult for a lot of players to
understand because you get these 5 points, but you don’t exactly know
where to put them or what benefit you’re getting with them. You might
make some obvious choices, for example, with Diablo II’s Sorceress,
you might put all of your points into energy because that’s the
obvious choice, right? Except that for almost every build out there,
you’ve just made the wrong choice. Any system where you have to go up
onto the Internet to figure out what the right answer is, is not a
good customization system. Any system where there’s a “right” answer
is not a good system for customization. The truth is, with stat point
systems, they are simple math. It’s not hard to figure out what the
absolute best choice is so we decided we didn’t want that as a
With that being said, we do have another system we’re working on. The
specific intent of it is to capture the imagination of what stat point
spending was supposed to do, which is, “I want to be stronger. I want
to be tougher.” These kind of simple ideas are not contextualized well
within a skill system. The skill system is about what the player is
doing, not higher ideals about what their character is. So, we’re
going to work on a system that really satisfies that feeling, but is
way easier to understand and also has some true customization to it.