I'm really not bothered that much if the 1st episode is a little on the weaker side. Honestly, I really wasn't that much of a fan of the original series' pilot, either, which felt a little slow and overly childish. The show picked up nicely by the end of the 1st season, though, and excelled in the 2nd (which is, IMO, the best season of the show). Korra will probably do the same.