It's been done several times, but since I'm a nerd who likes to delve into this stuff, I'm going to give my wall-o-text as to the lore-legitimacy and gameplay mechanic understanding behind some of the stuff listed in this "confirmed" information (looking forward to Blizzcon updates).
Increasing the level cap to 85 is completely viable if the leveling itself from 80 on is revamped as much as the rest of the old world looks to be. Either leveling will take longer, it will be more gradual (maybe gain new abilities in between levels or some such), or be fairly quick, but each level granting a lot more to the characters (extra talent points, or something more?). Blizzard has almost complete control over how things run, the only exception being stubborn programming and unforeseen complications.
Onto the classes, most have been fully explored and explained, but here's a compiled list of the best explanations I've come across:
Human Hunter- This is an easy one. Human riflemen litter the first camp at Northrend, and Nathanos Marris (see:
http://www.wowwiki.com/Nathanos_Blightcaller) has proven that humans are capable of mastering the art of being a hunter. They also have night elves, dwarves and draenei that can help them ease into it.
Orc Mage- There isn't a lot of lore backing this one up, but frankly (look out, it's my opinion, not solid fact!) it doesn't really need any. To tap into the arcane is, based on everything so far in the lore, merely a question of patience and intellect (and considering non-Ogri'la ogres manage to be mages, not a lot of the latter is necessarily required). Honestly, the only class lacking a mage in this list so far, the tauren, could just as easily tap into it, but they are a spiritual race, and choose not to. Also, the threat of being turned into sheep and fed to orc hunter pets might get those lazy peons off their rears.
Night Elf Mage- As many have said, the night elves, or Kaldorei, were originally arcane users who tapped directly into the Well of Eternity before the old Sundering of Kalimdor. There are still Highborne (members of the Kaldorei once considered as upper-class) that exist who are basically night elves that use arcane magic. In desperate times such as those in WoW, the Darnassians could turn to their exiled kin for assistance. There's even a new quest chain that hints at this. Frostfire bolt never looked this sexy.
Dwarf Mage- During the original WoW beta, this was already a playable class/race combo, and like orcs, not a lot of lore has to directly support this. If the stubbon dwarves decided to look into the arcane, by Muradin's beard they'll master it! Now pass me another pint! I'm gonna try this polymorph thing on a gnome and see if it makes a wee sheep out of 'em!
Blood Elf Warrior- These already existed in previous Warcraft lore, and there is no reason that anyone from any race can't be a warrior. It was only taken away from them for balance purposes in the first place, from what I understand. Players better keep show helm turned on, though. Need to protect that pretty face and hair while being bludgeoned to death.
Dwarf Shaman- Another race/class combo that has been fairly covered; an entire society of Dwarves known as Wildhammer already ARE shamans. Does anyone really think it's a stretch that some mountain dwarves wouldn't learn from them, especially considering their growing interest in their own history? Stone form and stone totems seem like a perfect fit to me.
Undead Hunter- As mentioned in my human hunter explanation, and considering that the undead used to be human, becoming hunters is a cinch. Their faction leader is a ranger (a "dark ranger", but I doubt there is too much of a difference) and one of her most trusted allies is a straight-up undead hunter with two canines. Pets running off with limbs aside, it seems just as viable as any other class they have available.
Tauren Paladin- There's a quest chain that shows tauren that are doubting the religious ways of their people, and looking to a new light besides moon light. Sounds like a perfect reason for them to jump on the pally wagon. It's not like their digging into fel or arcane, naysayers. Becoming a disciple of the Light is a very spiritual path, and the tauren have always been a very spiritual people. Like the Blood Knights before them, tauren paladins will likely just find a different path to the same end. I'm going to get tired of the Holy Cow jokes reeeeaally quick, though.
Tauren Priest- This one is a no brainer, and that the race has lacked priests this long can only really be attributed to old class balances. The tauren faith has always been exceptionally strong, and the power of a priest is based on belief and faith. Considering they don't currently have a clothy race, though... do they make dresses in an XXXXL? I don't even think a Draenei robe would fit.
Gnome Priest- A lot of people cast doubt on this one, and for good reason. The gnomes have always been mentally attuned to logic, reason, and science. Tinkering with mechanical gizmo-whatsits is their claim to fame, more so than the economic-minded Goblins (though they are equal in their ability to blow themselves up with their own gadgets). However, their faith has probably grown exponentially after having seen their triumphant allies pull through in some of the most impossible situations, and managing to get their home back would only make their faith in themselves stronger. I wonder if their patron god will be Mecha Zod?
Troll Druid- Just like blood elf and tauren paladins, the potential for a troll druid makes a lot of sense. They don't necessarily have to specifically work with the Earth Mother, Cenarius, or any prior druids to be labeled as such. Delving into their personal worship of animal gods could easily lead them to nearly identical abilities (such that have been displayed by trolls numerous times in the past), even if the source of the power is different. Certainly we might see some very different forms from them (Amani war bears, tigers, and the like), but that doesn't discount that they could work exactly like a druid, and possibly teach their tauren brethren a thing or two.
With the new races, I thought the original post gave a solid reasoning for both. The worgen NPCs that players have hunted all this time are certainly completely feral, but if the isolated kingdom of Gilneas has found a cure for the bestial insanity, it wouldn't be surprising that they'd be scrambling for allies once reunited with the outside world. Who better than their old "buddies" in Stormwind? Certainly, there will be a grudge considering Gilneas' original reasons for ditching, but with the world going through yet another massive catastrophe, such old issues might be water under the bridge.
Goblins joining the horde seems like a no-brainer for people who have been playing Warcraft games since 2. Besides that, the Goblins have always favored the horde. They will gladly sell something to a human, but do you see them providing free zepplin rides to major alliance cities? They are a natural counter to the gnomes; both races are very shewd, interested in technology, and act rather silly and peculiar at times. Also, if several of their neutral towns are laid waste to by the new cataclysm itself, they're probably going to be looking for a few couches to crash on.
The choice to revamp the two huge continents of Azeroth is a huge plus for me, and I think the next logical step. In order to complete these worlds, hundreds of design choices will have to be changed, both to make the world graphically fit with the newer content and to avoid clashing with flying mounts. All of the zones are likely going to be overhauled to match up with the cataclysm from zones being gone completely, to others being shattered and unrecognizable. Finally, several places that previously were locked out from people on foot will finally be accessible, including major instances people have been pining for since early vanilla WoW. Besides all of this, the Maelstrom will probably not be neglected, considering that the source of the world-shattering event is somewhere in their, where Queen Azshara and potentially Deathwing awaits (unless Deathwing just heads back to Dire Maul, where he's apparently been this whole time).
There's a lot here, and just because it's in the same space as old content doesn't mean it won't be new. These zones look to be getting full-on geographical changes the likes of tsunamis and earthquakes over the entire world. Sounds pretty fresh and new to me.