I'm more of a casual player now than I used to be, but I've always tended towards more of an altaholic than a hardcore raider. The problem with playing alts used to be that you gave up all the stuff your high level toons had, and if you wanted it again you had to regrind all of it. But WoW as it is right now, it's so damn altaholic friendly that there almost isn't any down side at all to playing alts. Account wide achievements, mounts, pets, and titles, pretty much if you earn it on one toon you earn it on all.
]And I'm loving this more than anything else in MoP.
Tuesday night I was grinding on my high level druid when I got sidetracked doing pet battles and leveling up my Blizzard Bear. Last night I leveled my new Panda monk to 13, but I also continued to pet battle my Blizzard Bear with any critters I came across. This morning I logged onto my 62 human pally, flew around on a mount (that my druid earned months ago), and yeah continued to pet battle that same Blizzard Bear on low level criters in an entirely different zone than I had the two days before. I even captured a few new pets there that I will be able to access back on my druid later tonight. And my pally got two new pet battle achievements, only it's not just an achievement on that character, it's on my account and every character now has it.
Yeah, new zones and high level content aside, the alt friendliness of WoW as it stands now is nothing short of a Godsend for me. It's incredibly awesome in ways I probably haven't even realized yet.