Always, the best time to start I reckon is when there's a new expansion pack. You will probably get some people who will level some 'alts' who might play with you. Otherwise it's kinda lonely unless you got a bunch of friends to play with you.

Currently all the starting zones are like ghost towns. When I first started out it was really an amazing experience with tons of people all over the place. Progressing to 60 with all these people and seeing them from time to time really made WoW one of the best MMOs I've ever played..