Bashiok, Diablo III community manager, said in a post on the forums that max Character Level is set at 60. This decision by the development team was made for many reasons, some of which Bashiok lists in his post.
We've been working with a level cap that's quite a bit different. It was something I had alluded to a while back but I think it's probably worth discussing sooner rather than later.
So, we're working with a lower level cap for a myriad of reasons but really the main point that they're more or less all linked to is that we came to the conclusion that a cap of 99 exists for the sake of itself. Meaning that it's a really high number despite all sorts of good things like meaningful player/skill/power pacing, item distribution, balance, etc. It's sort of this long term goal that really didn't matter, almost an Achievement without the flashy graphics or *bwong* sound. You didn't need to reach 99 to do anything, in almost all ways the game stopped somewhere in the 80's (for a lot of people a lot lower), but by stretching the player out over an additional 20 or so levels to 99 it created all sorts of issues we were having problems justifying forcing the game in to. So we're working with a level cap of 60 at the moment.
This is the point where some of you scream "60!? That's like original World of Warcraft!" And then the rest of you /facepalm and say "60!? Couldn't you pick any other number? Now everyone is going to compare it to World of Warcraft!"
The 60 level cap we're working with came from a lot of time and thinking about our content and how we want the experience to feel at every level. It so happens that it works for us really well.
Of course it all comes down to an XP curve. We could, for instance, say the level cap in Diablo III is 60 and then pace that curve and gain out over what we estimate it took someone to reach 99 in Diablo II. Of course we wouldn't do that but it should help illustrate that the time from 1-60 in Diablo II does not equal the amount of time it will take to reach 60 in Diablo III. 60 levels versus 99 levels doesn't mean less content or less powerful characters, etc. These aren't uniform levels of power that move from game to game. And in fact we are pushing a longer game than Diablo II and I'd argue our characters feel way more powerful. Ok, that out of the way, moving on!
The leveling experience is always going to stop somewhere because the real game is the item hunt. So, instead of letting it drag out to a less meaningful 80 or so levels like most people saw in Diablo II we have 60 levels of awesome; at every level you'll get a meaningful and noticeable increase in power. It has a ton of other benefits and fixes a lot of problems a higher cap causes, but I'll take pause.