As you know, the new race and class combinations coming up in the Cataclysm will open a whole new set of doors to people who want an alternative character choice that goes against the grain of their typical racial customs, to one degree or another. With the exception of a couple combinations that feel as though they should have been there from the beginning (such as blood elf warriors, which need no discussion here), each new possibility presents roleplayers with an opportunity to play an outcast of sorts, a character who has made a significant break from the traditions his or her race usually represent.
The lore behind each combination is not yet fully clear. We know tauren paladins will probably worship the sun and call themselves "Sunwalkers" for instance, but not much more than that. Some things are clear, though, and there's a lot to get the imagination going for those roleplayers who yearn to play something a little different.
Tauren paladins and priests
Here we have the most obvious departure from the traditions and culture of a race in WoW. Blood elves already had a somewhat convoluted path toward becoming paladins at first, but their story really played out throughout the unfolding of The Burning Crusade expansion, and eventually ended in a redemption that helped make their status as paladins seem much more reasonable. Still, even if you discounted all that, you could argue that the blood elves had once worshiped the light, so it made sense that they could come to do so again.
But the idea of tauren becoming followers of the Light seems absurd at first glance. The good thing is that we're pretty much certain that the tauren paladins will not be Light-worshippers as such, not in the same way that humans and draenei are. Rather, somehow a new perspective on their ancestral connection to nature will open for them a new sort of path that just happens to bear an uncanny resemblance to the paladins of the Light over on the Alliance side, much like priests of different races have different beliefs, yet share similar powers. Whatever it is will probably make sense for tauren paladins and priests alike, but will probably still be a sort of fringe culture for tauren, much like shamanism has been for draenei.
This option seems to be the craziest of the bunch at first glance, but when all is said and done, I suspect that tauren paladins and priests will be more connected and unified with the rest of tauren culture than we would expect by just looking at the WoW lore as it is today, before the Cataclysm strikes. Taking up sun-worship may be more like the dwarves discovering their ancient roots as special creations of the Titans than a true revolution in beliefs or values.
Dwarven shamans (and mages)
Speaking of dwarves, mages are one of those classes that they probably should have had from the beginning -- but shamans, on the other hand, may prove to be something more interesting. Shamans may not have all that much connection to the Titans as such, but they are very connected to the earth element, which the dwarves now know themselves to be born of. Some dwarves of the Wildhammer clan are already shamanistic to a large extent.
Nonetheless, shamanism is something new to the general population of dwarves. Whether or not dwarven culture as a whole embraces it, it will be something most dwarves are not very familiar with, and didn't experience much growing up. I imagine it would be like the relationship between city-dwellers and rural farmers, who may generally belong to the same nation and culture, but maintain very different lifestyles within it.
As Michael and Daniel have already noted, there is a precedent for trolls shapeshifting and taking on animal forms already there in WoW lore. What isn't there is any sort of relationship to the moon goddess Elune and her son Cenarius, which normally forms the foundation of druidic culture. The likely connection between the night elves and the worgen indicates that worgen will have some sort of relationship with Elune, but there's nothing hinting that trolls would have one in the game at this point.
I always thought that druidism came with its own set of beliefs related to Cenarius, as well as protective attitudes about nature that I haven't seen trolls express so far. Could it be that some of the darkspear trolls are turning away even more from their vicious and barbaric past, and following the tauren druids in more constructive efforts to heal the world?
On the other hand, Michael and Daniel were thinking that trolls own belief system related to the Loa spirits could give rise to the same basic set of abilities as Cenarius-based druids. If so, this would create a "two religions, one class" type of system we will probably see emerge with the tauren paladins and priests. If so, then troll druids wouldn't be so much of a departure from their culture at all.
Human and forsaken hunters
Human hunters are likely to enter the game without any extra introduction from the new lore of Cataclysm, except maybe a bit about how the great changes in the earth have forced them to use more hunter skills in order to survive in treacherous places. Still, human hunters have been around for a while now, and it only stands to reason that they should be available to players.
Forsaken hunters have likewise been around for a long time -- Sylvanas herself is a "dark ranger," but in this case there might be an extra focus on a new regimen of dark rangers in Sylvanas' army. Far from being outcasts in their culture, forsaken hunters could be seen as their people's most elite fighting force.
Night elf mages (and orcish ones too)
Night elf mages are probably the most like proper outcasts among all the new combinations, since they openly practice something their race has sworn to abandon. As Metzen mentioned at BlizzCon, they're also a good opportunity to play a really ancient character, if that's what you'd like to do, since young night elves probably wouldn't have had much opportunity to study arcane magic lately. So if you're going to be a night elf mage, keep in mind that your character has probably been very solitary for a very long time, unless he or she would have studied either in secret, or in exile.
Orcish mages are an odd option because there's really no lore for or against them -- how orcs could come to be mages isn't really clear, but nor is there any reason why they wouldn't. Until some sort of story appears around them, it's hard to know how to roleplay one, except possibly as a former warlock, who has turned away from his or her demonological ways.
And last but not least, thinking of gnomes as priests brings to mind some exciting possibilities. I am thoroughly intrigued by the ideas Michael and Daniel had about a possible "Church of Innovation and Discovery" -- that a gnome could cast divine healing magic out of his or her faith in the constant exploration of new ideas seems perfect for their race.
Gnome priests wouldn't exactly be "rebels" as such -- gnomes don't have anything against religion as such, but up till now, the structured beliefs of other religions haven't really made sense to gnomes. Gnomes do have a lot of faith, however, especially faith in possibilities -- if you ask a gnome whether a problem can be solved or not, undoubtedly they would say yes, as long as they have enough time to figure out some sort of answer.
Still, gnomish priests clearly represent something new for their people, and an ambitious roleplayer could play one of those gnomes who thinks he knows what religion is all about but doesn't, constantly praying for the next boss to drop good loot or whatever, winking at his Light-worshipping friends as if they share a secret together.