You're getting better tire selection and maybe better wheels but that depends on what you already have. There's nothing that fundamentally makes 700c better, it's just the new standard.

The other benefit is that you'll get more tire clearance so you could potentially fit a wider tire than you could before if that's something of interest. The only downside I can think of is that the bike might "look funny" with the slightly smaller wheels than it was designed for but I really doubt it would be noticeable.