It's been confirmed that Warlords of Draenor will come with a free boost to level ninety for a single character - in fact, you'll get it even before the expansion actually comes out, as long as you pre-purchase. It's also pretty much a given by now that at some point afterwards, level ninety characters will become available for purchase as an out-of-game service, similar to server transfers and race changes.
Both Liore and Wilhelm have spent some time pondering this week which class they'd like to insta-level this way. I did a bit of thinking about it too, but the end result was pretty much that it's a moot question for me. It's kind of funny actually: before resubscribing in December, the idea of a free boost to ninety with Warlords of Draenor sounded great. I pretty much shared Nils' mindset: "let me skip the silly panda expansion, thanks". Now that I've actually played MoP though, I think that pandas aren't so bad, and I wouldn't mind levelling another character through Pandaria. And I don't inherently prefer endgame play to levelling, so it seems pointless to skip half the fun.
I've seen people suggest that you could use the boost to level up a class that you previously struggled to level up. In my case that would probably be the warlock. For all the alts that I made over the years, I still only have two warlocks: one is my bank alt on my old Horde server; the other was my very first alt ever, created back in late 2006, and he's only made it to level 44 during the last seven years. The thing is though, I don't think I've ever struggled with the class due to anything inherent to levelling it - 'locks just haven't "clicked" with me. I don't see that changing just because I suddenly have more buttons. So that would be another pointless boost.
I think I'll just save it in case some sort of rare and special opportunity arises where boosting a character to ninety would actually make sense for me. Like, I don't know, a sudden desire to raid with a friend on another server where I don't have any characters. I don't think it's likely to happen, but there you go.
What I find a lot more interesting is the question of how making insta-90s available for purchase will affect the game as a whole. I think from Blizzard's point of view it's going to be a big win, at least in the short term. It's going to be another thing that they can charge big bucks for on top of the subscription (can you really see a boost to ninety for an entirely new character being cheaper than transferring an existing one?), with the only downside being the risk that some people will cancel their subscriptions earlier than they otherwise would've done, purely due to this new feature. And I don't think there will be very many of them. I suspect there will be some players that will "binge" by buying multiple 90s and will thereby burn themselves out more quickly than they otherwise would've done if they had been slowed down by manually having to level each character, but I can't see them making up a significant portion of the player base.
The question of how insta-90s will affect the players is going to be a more difficult one to answer. I suspect that as a tool to get lapsed players to return and jump right into the new content it will be pretty successful - though how well those players will be retained after their first month will depend on how Warlords of Draenor actually plays.
There will also be new players for whom it will be a boon that allows them to jump right to a level where they can play with their friends' already max-level characters. Like the binge buyers, I don't think that this group will be all that sizeable however.
To most long-time players, I think it will be just another thing to spend money on that might give them brief joy, but that isn't really a game-changer in the long run. (I'm thinking of all the people I've known who've server-transferred, faction- and race-changed about half a dozen times by now.) Another character at ninety will sound great in theory, but in practice it will be just another toon to run dailies or raids with, and without having invested any time into levelling, I suspect that many players won't actually feel that attached to those new characters. It reminds me of when SoE announced that they were selling high-level characters for Everquest 2 last October. Several bloggers I read immediately jumped on the bandwagon there too, talking about how they always wanted to have a look at higher-level content in Everquest 2, how they had always been deterred by all that levelling and this was just the thing for them... just to abandon the idea after one or two play sessions.
There will be a dark side to the insta-90s too however: unless Blizzard puts some kind of restriction on who can buy them (which I doubt), people who are completely new to the game will buy them to play with the "cool kids" right away... and they will be terrible. I've seen people say that levelling doesn't teach anything to anybody anymore, and that anyone could pick up a new class at ninety and master it within the hour. That may be true for an experienced player, but for the truly new to the game... nope. I vaguely recall running a Drak'tharon Keep pug back in WOTLK with a paladin who seemed to have absolutely no clue what he was doing, what any of his buttons did, anything at all - and eventually he fessed up that he had just bought the character on ebay (or wherever). That's the kind of gamer we'll all see a lot more of once insta-90s become available for purchase. I'm not saying they'll be ubiquitous, but there'll be enough of them for it to be noticeable, and they'll make the worst players you've seen until now look like superstars in comparison. "You bought that character, didn't you" will become the new insult of choice for anyone who doesn't know how to play.
And randomly assembled groups will be worse than ever.