You'd be skipping the previous 8 years of content. How much that means to you is variable.
Right now 1-60 is just over 4 years old. 60-70 is 8 years old. 70-80 is 6 years old, 80-85 is 4 years old again, and 85-90 is 2 years old.
What this means is, as you're levelling you get pretty noticeably different styles of content depending on the era it was added, some of it isn't as newbie friendly as others, if you just do quests and keep your head down though, it basically comes down to most of the stuff that is 4 years old or less being pretty well guided, quests will have an organic way of leading you around (though you may outlevel them quickly, there is WAAAAAAAAAAY more content to level through than you will ever see on one go, the opposite of most MMOs), then you hit the older stuff and it's just kind of quests randomly strewn about with much less coherent storytelling.
Even some of the newer stuff (Cataclysm and Mists stuff) has pretty compelling stories but the quality and how much it interests you can vary from zone to zone, which I think is sort of inevitable given the style of game. Compare it to something like FF14 or SWTOR and wow doesn't have any overarching story you're constantly doing, but the zones you're in have lots of self contained stories that contribute to the world building. If you pay attention you'll easily start to piece together everything that is going on, but it's certainly not all as interconnected or overt.
Contrast that with the new expansion where, the zones still have their stories and those are cool, but there's also a VERY clear progression on top of it against the iron horde. Mists was sort of similar but not quite as integrated with all aspects of the levelling experience.
I still think as a new player it's worth it to go like, 1-20 or maybe 1-30 (miinimum, if you're enjoying it just keep levelling... it's not a race) to get a feel for the game and how it plays, maybe make a more informed decision on class at that point, make sure to run some dungeons in there see how that sort of experience goes for you. It'll tell you A) whether you like the game at all and want to plunk down some more cash. B) HOW the game works period, the 90 boost isn't super intended for brand new players, it's not difficult content or anything, but that's from the perspective of a veteran. I have no doubt it could be pretty overwhelming to someone brand new even just understanding HOW to effectively do quests and fight mobs, let alone with 15-20 hotkeys.
There's a beginners guide over on the official site, as well as a new players forum (I have no idea how helpful either of these things are, but they're there):
http://us.battle.net/wow/en/game/
http://us.battle.net/wow/en/forum/1011644/
But we're a good resource too if you have specific questions. Class choice? Just go for what sounds fun. We could definitely get more nuanced about them but initially know that what you envision in your head MAY not be exactly what they're like after 10 years, which can put some people off early. There's the fantasy of "Yeah I shoot arrows and have a pig buddy" and then there's the reality of "I am the spammiest class in the game" for example with hunters (Don't get me wrong, my hunter is my favorite alt). Wow is a deceptively simple game, all of the classes have a LOT more depth and nuance than is first apparent and it is not an easy game to be great at, it is an easy game to be GOOD at.
