Playing with friends is probably the most fun you can have in WoW-- while it's fun to solo, a good group is where it's at, and there's no better group then playing with people you know. Unfortunately, because even people who have a lot in common hardly ever level at the same rate, level differences grow quicker than expected, and pretty soon it's just not possible to play with friends without someone wasting their time.
Until friendship shows up anyway. Lev over at WoW Ladies writes about an interesting duo she made with another player-- after joining up for a quest in Winterspring, they've rolled 10 levels together, and even played on their alts. And the weirdest thing is, she's 22 and he's 14, so you don't think they'd have a lot in common, but she says they play together great (she has a younger brother, and she compares their relationship to that). In the comments, someone about her age confesses to being grouping buddies with an 80-year-old person, and someone else tells a story of leveling up 40 levels with someone they met in game.
Makes me kind of jealous, actually-- I've played a long time with the same people at 60 (and now 70), but leveling, I was never at the right rhythm to really grow with anyone else, either in common or otherwise. On my very first character, I played a night elf hunter (duh), and got some help from a night elf priest on how to start playing the game. I still have that guy on my friends list on that realm, and still see him play online, but he outstripped me in terms of levels a long, long time ago (he's 70, and my hunter is stuck in the late 50s). Now, it'd be nice to somehow technically be always able to play with friends (you could have instances that averaged your levels, or just play with premades, which is what I think Guild Wars does), but I think that's just one of the drawbacks of having a leveling system-- unless you only play those characters together, someone will always fall ahead or drop back.