Creative comes first, and it's top-notch.
Blue-chip clients: where else can you work on IBM, GE, Converse, Nike, Pepsi, McDonald's, Motorola, Harley-Davidson and Target in the course of a few years?
Talented, hard-working, professional people. You'll learn a lot from your colleagues.
Freedom to be yourself. Relaxed work environment. Personalities – more... welcome.
Work-life balance, though improved in the past few years, is still heavily tilted in favor of work.
There's not much of a career path or mentoring to speak of. You'll learn a lot from doing the work, and working with smart people, but you're making it up as you go along from a career development standpoint. (This is especially true if you're not a designer.) – less