price. for starter lenses, you could get the 50mm 1.8 (great little lens and its functionality WAY outweighs its cost), a short lens, a general use walkaround lens, and a decent telephoto, all for realistically about $600.
and I'm not talking about cheap junk. any of that money invested can likely be recovered come time to upgrade too.
edit: actually, fitting a short lens into that budget may be rough... as from what I can tell they tend to be the most expensive type.
short lenses aren't cheap, but the long lenses are WAY more expensive. look at any of the long canon primes. Huge money.