You can also level doing more than PvE. For instance, I have a 19 rogue that I leveled from 10 to 19 in pure PvP. (750 xp / turn in, 3 losses or 1 win per turn in...) Getting about 1,500 hk's or so a day... yea... some would get bored. Not me, I say, backstabbin them crazy horde lowbies beats all the PvE in the world! I thought I would have made knight by now, but the bar is almost there (bastage blizzards and their slow hk's for noobs!) Would be nice to hit Knight-Captain lol
Also, in PvE, you do always feel like your gaining. Even fighting fast kill greens moves your bar *The advantage of having such a long bar) Even for casual gamers, who get rest xp. I leveled my Paladin, priest, and hunter to 60 from 59 in the same week (I'de play 1 character every day, then the next... good rest build up and it doesn't get boring playing same areas with different abilities. Just my two cents ^^