The people. You are never going to be around this many fun, smart, awesome people in any other firm. It's what makes the hours and the **** days manageable. We also work with clients from all ranks in their firms, not just focusing our time/effort on the C-suite. I really like working with someone my level (just as much as I like presenting to the CEO) because I know my work is more likely to make an impact when everyone in bought-in. We're not stuck-up.
The inconsistency of hours / work-life balance from office-to-office. It's well-known that some offices are more conducive to having a life than others and some partners are more sane than others (for lack of a better word). After two projects with crazy partners I know I'll be seeking out the ones with a reputation for achievable standards.