"This is pay-to-win!" - I guess it depends on how the biased mind looks at it, but there are several reasons why no one who uses the free boost or purchases the paid one will "win". The boost offers no advantage to someone who uses it over someone who doesn't, other than they opted to bypass gate of tedium that is five tiers of content. No one "wins" by trudging their way through five tiers of broken and cut-up nerfed content. When you can move on to the next tier without completing even half of the xpac you're currently leveling through, where is the "win"? Where's that sense of achievement that you would have gotten had you been there at the time that it was relevant? People's comparisons are tantamount to saying that it's like allowing a person to start at the last leg of a race. That's not totally accurate. It's grace. It's allowing people who got there late the option to pay a fee to start at the stage of the race where everyone else who got there on time has rested up and is waiting to start again. The part of the race that everyone is actually watching. Anything before is useless and irrelevant. It might be a pretty sight, sure, but it's useless and irrelevant. There's no "win" in doing it, or skipping it.
It's not a boost to 100, either. No one is being allowed to skip anything that's relevant. Someone who boosts to 90 will have just as hard a time as someone who didn't when they finally get there, and they'll be at a disadvantage to the people who got there first (i.e. the people who raided at least a little bit in MoP). The gear a freshly made 90 will be in will likely be 384, no higher than 425. Gear for the leveling 90, not the raiding 90. It will be harder for the boosted 90 to survive in Draenor than you, if you did anything endgame in MoP. How is that pay-to-win by any definition? It's pay-to-compete, and just barely that.
"This lets people skip too much of the game!" - I've seen stupid numbers trying to portray the percentage of "content" that is old-world stuff, as high as 95%. That's just not true, regularly. I don't have a solid number, but I doubt anything before WoD when it comes out will be even 50% of the game. You don't have to do half of BC before you can move on to Northrend, even if you only do most of the dungeons once. You don't have to do a third of Northrend on a one-dungeon basis. To my knowledge, Cata is the only expansion content whose leveling wasn't nerfed, and even then, you don't have to finish a zone before being able to move on to the next. (You can move on from VJ and Deepholm at the halfway points.) Depending on whether or not you were in Krasarang, you can hit 90 today at early-to-mid Kun-Lai, when in the beginning you would cap mid-Townlong or mid-Dread Wastes, again, doing every dungeon only once.
Unless you go through the immense trouble of actively and carefully freezing your experience, there is no possible way that someone who buys WoD is going to get the maximum value from old content, if any value. It's not 90% of the game, especially if you have multiple characters, and the prices of old expansions today reflect that.
"People won't learn their class! I'll be surrounded by (more) noobs!" - This has always been the fault of players, not Blizzard. If people don't learn the basics of playing their class and role after Blizzard and others goes through the process of supplying them with ways of teaching them (Proving Grounds, core spells tab, WoD intro questline, Icy-Veins, Noxxic, etc), then Blizzard is not responsible for their badness. It is a sad truth, though, that there will be people who will still suck at this game even if it does do everything it can to teach them. Can't blame that on this feature, though. Blame it on dummies who can't learn, like we do today, and will continue to do in WoD, with or without the boost.