I found this over on Tobold's site, and it purports to be leaked information from a play-tester explaining exactly how Hero classes will work. The explanation is similar enough to how classes work now--and it's kind of unimaginative and dull--for me to believe that this is how they will be, or at least very close to how they will be.
Here's a snippet:
Firstly each of the nine base classes can branch out to exactly three hero classes. For example, Druids can branch out to Druid of the Talon (nuking), Druid of the Claw (feral), and Keeper of the Grove (healing), while Paladins can be Champion of the Light (healing), Mountain King (defense, and yes they twisted lore here), and Knight of the Silver Hand (offense). More details on specific hero classes later. As you can see, each of the three hero classes plays to the strengths of that classes' three talent trees. However the main purpose of Hero Classes is to allow players to tangibly improve their characters at max-level through means other than gear. Since Hero Classes will be released after The Burning Crusade, they will be obtained at level 70. Surprisingly they'll not only be available to every player, without a quest, but they'll either be trainable for free at the corresponding class trainer, or require a simple and straightforward quest. Blizzard's reasoning is that Hero Classes should be a natural progression and reward for all players who have achieved max-level, rather than a grind for only the "hardcore".
There's two catches though: 1) specializing in a Hero Class is like specializing in a profession - you're stuck with it forever, and 2) you won?t immediately have access to all hero skills. Think of it as starting a new class - you start off with a couple of base abilities, you get a few more from class quests, and the rest come from your class trainer. The same applies to hero classes; picking a Hero Class will start you off with two new hero skills, and the rest will come from hero class training (called "hero talents") and hero quests. Here's the interesting part: hero talents uses Hero Points, rather than currency or regular talent points. Blizzard hasn't finalized how Hero Points will be earned, but they will most likely be obtained through a combination of experience/honor, and unique items that give you one or more hero points when used. Most likely those items will be given as both quest rewards, and drops off legendary mobs like Illidan and outdoor raid bosses (emphasizing the "heroism").
It sounds like it will lock you into one role pretty hard and also perpetuate the boring (to me at leats) Raid-or-Die paradigm. Yay? Click through to see the big list of what spec becomes which Hero class.
(Via Metroblogging Azeroth.)