So I decided to roll a low level character and level him without heirlooms on the beta, just to try it out for myself. And so far, I feel like they've made pretty decent strides in balancing things out - I'm at level 7 now after a half hour and I could probably get to ten after another 20 minutes. I am, however, concerned with what Kalyeatrish says about the level 8-9 relative weak spot in leveling. If players can't get stuff done at those levels, it'll be a hard slog to get to 10. And most people who try out WoW don't get past 10 - that's a barrier to entry you don't want to imbalance when you make game changes.
Hopefully it's something Blizzard keeps watching, and I hope everyone in the beta manages to find time to roll a character and test it out at low levels - it may seem like a waste of time, but it's actually incredibly important for the game long term.