Despite my dislike for the religion fluff, I'm actually excited about the potential of D&D 4. I still haven't seen it in action, but the character build options have captured my imagination. The standard classes are fine; fighters, clerics, wizards and rogues look promising, although the ranger looks dull to me, but that's not new. The classes that have me excited are the paladin, warlock and warlord.
The paladin is a holy warrior, a malleable class that no longer must be lawful good or follow a strict set of guidelines. Time will tell if their combat prowess is up to snuff; the 3.5 paladin was a disappointment, along with the horse he rode in on (thankfully gone). The concept that has me excited is playing a narrowly focused concept paladin. For example, a paladin of Ioun, god of knowledge, might be devoted to seeking out and destroying those who dabble in forbidden lore (maybe even warlocks). A dwarven paladin of Erathis, god of invention among other things, might be a seeker of lost inventions, considered holy by his faith. A paladin of the Raven Queen might quest to destroy necromancers, who defile the dead, the core concept of his deities portfolio. Each of these paladins could have an interesting flavor and a focus other than bringing justice to evil doers.
The warlock is slightly less sinister than its 3.5 version, although it's certainly meant to be walking a fine line. The concept that captured my imagination was the bookish warlock, scholars who believe they can use forbidden secrets for a greater purpose. I imagine a Giles like character from Buffy the Vampire Slayer, librarian by day, demon hunter by night. Perhaps there's a secret society dedicated to destroying infernals by channeling the energy of otherworldly beings, or vice versa. Perhaps the devils and demons are so afraid of Cthulhu like outsiders that they've entered an ancient pact with humans to fight them. With just a bit of flavor text, the 4.0 version of the warlock seems more interesting and more accepted in D&D society than the 3.5 version.
The warlord is a battlefield master, a support character that can assist in combat by moving the party and monsters around like chess pieces. Some have said this is the perfect class for the player who likes the bard, a helpful sort who can provide battlefield bonuses to the party. Others say it's perfect for the micro-manager, who pulls his hair out as he watches the party make dumb tactical decisions. Don't like what the party is doing? Shift them around. I would much prefer that the helpful player take up the warlord. As a DM, the warlord worries me, as it's new and I have a hard time envisioning how its powers are employed without the game turning into Warhammer or chess. A more timid DM might ban the class, but I'm curious to see it in action, especially employed by a thoughtful player.