I JUST (as in less than thirty minutes ago) hit 90...on my 10th toon. I also have two toons fully ready and geared for raiding (one main, one alt) and several at various points in LFR-progression.
Leveling can be a bit "grindy" and boring after the first two. However it is by no means unfriendly to alts. Especially considering the various ways Blizzard has tried to ease alt-progression with things like BoA rep boosts, increased LFR drops, ect.