From Diablo Wiki
Unannounced Class Progress
Jay Wilson gave a report on the progress of the 4th and 5th characters in a December 2008 interview with 1up.com. 1UP: Can you talk about the state of the classes? For example, is every class -- even the two that you have yet to reveal -- playable in the game right now? Jay Wilson: No. The fourth class is playable in game but is using a placeholder model -- one of an NPC, actually. And all of the skills are what we call "programmer art" because we haven't implemented actual skill effects. So that one is just in gameplay testing for us while we determine signature skills. The fifth class, we're just about finished concepting it and are about to start building it. We can't use placeholder art, so we're now just going to build a model and start working on the first series of skills for that one. We're actually spending more time on the Witch Doctor, Wizard, and Barbarian because of a big change we made to the skill system that we wanted to [integrate them] with -- so it's mostly just artwork that's going on for the other two classes.
It's not clear why they couldn't use placeholder art for the 5th character, but that's something fans are enjoying speculating about.
 Known Class Information
The D3 Team has confirmed that the Barbarian will be the only recurring character class from Diablo II,of the original 5 Diablo III characters. More characters will be added in expansion(s) and some may be revisions of earlier character types. We also know that the Diablo III dev team at once thought about making a Dervish class, but no information if that idea is completely shelved. It seems highly unlikely that the Dervish would appear though, as Blizzard have avoided talking about upcoming classes, and they would be unlikely to mention the class in passing if that is one class that will make the game. The Barbarian is the only Diablo II character to make it into Diablo III release. He's quite updated in style and use. The Witch Doctor is a new character, similar in use to the Necromancer. The Wizard is the third announced class, filling the archetypal magic user role previously occupied by the Sorceress. The Monk is the fourth class, and is comparable to the monk from the Hellfire expansion. The following characters have been confirmed to NOT appear: The Amazon will not appear in Diablo III. The Assassin will not appear in Diablo III. The Druid will not appear in Diablo III. The Necromancer will not appear in Diablo III. The Paladin will not appear in Diablo III. The Sorceress will not appear in Diablo III. Some of these seem likely to join in on an upcoming expansion, that has been hinted about . The conceptualised Dervish class will likely not appear in Diablo III.
 Classes NOT Reappearing
Jay Wilson said the following in an interview:  The Barbarian is the only returning class. Originally we actually planned to have no returning classes, but as we developed the barbarian, it essentially ended up turning into a barbarian. We sort of got to the point where we went – because we actually had a different name for him originally - I think were going to call it this other name and everyone else is just going to call it the barbarian, so maybe we should just call it the barbarian. One of the things we really did not want to do – it's why we set that initial goal to not repeat classes – was, we didn't just want a rehash; we wanted to make the sequel and the sequel has new gameplay and new experiences. I know a lot of people really love the classes in Diablo 2 and it's not my desire to intentionally arc those people – I like those classes to; I love the Necromancer, he's my favorite class from Diablo 2 – but as our goal we really wanted to provide new experiences; that was our focus.
Jay elaborated on the issue of returning characters in an interview with 1up.com in December 2008. Jay: I don't want to close the door on bringing back classes from Diablo or Diablo 2; I just don't want to do so with the initial release of Diablo 3. In Diablo 3, all of the classes should do something completely new. Then I'd like us to look back and figure out what gameplay gaps [exist].... Then we'd go back and satisfy our and the fans' nostalgia by pulling back some classes that we think really stood out.  Speculation
Blizzard replied to questions as to why the Necromancer was not included in the line-up, saying it's so similar to the Witch Doctor. Jay Wilson responded that the Witch Doctor shall be made enough different from the Necro that if they decide to bring the Necro in later, their roles will not overlap. Here is a quote on class design choices by Jay Wilson:  We have other goals, where we want each class to be distinctive from one another, we want archetypes to be very strong—I should be able to tell you the name of a class, and you should know what it is, right away. You should have an idea of the kinds of things that that class can do.
It is likely that the fifth and last class will be a ranged class, in terms of what roles are needed.
 Fan-Made Classes
Fans have not just been waiting around for the next class, they have been busy making suggestions for Blizzard to consider. These fan classes include everything from a Flagellant to an Arcane Warrior.  Wizard Hints
The third class to be revealed was the Wizard, who debuted at Blizzcon, October 13, 2008. Jay Wilson hinted at the Wizard in advance, in an interview with Videogamer.com:  All the barbarian players are delighted and all the necromancers hate us. I understand, I don't begrudge them that. I would hate me too! But what I would say is that when we announce the next class, which is quite similar to a previous class, then all those players will hate us too. You can't make everybody happy, but I think when the game finally comes out players will find there's a good class for them, one they will love as much as the ones that came before. And if they don't, I absolutely promise that in the expansions we'll consider bringing back old classes. We just don't want to do it with the first release. We want to establish our identity.
 Next Revealed Class?
Blizzard has not yet given any hints about the fifth and final character, which we'll see in the initial release of Diablo III. Fan consensus was that there would surely be a mage-type character, with various sorts of magic for offense. The Wizard filled that pedigree nicely, but also removed the only widely-agreed upon character type. It seems certain that Blizzard will include an archer type character, one who specializes in the bow and perhaps other ranged weapons, but what fiction or other sort of skills this character would have to compliment his/her bow is open to wild speculation.
See the Other Characters and Skills forum for discussion of/educated guesses about this topic.  Additional Information The Diablo 3 Basics article lists all known features and changes in Diablo III compared to Diablo II The Media Coverage has all the various interview, previews and articles about Diablo III from around the web.