One simple but obvious feature they have is the ability to create DNS presets and apply those for new domains, it's really useful. (other people probably offer this)
Gandi also has fantastic support; it is slightly sad that they do not have a telephone number, but I get the vibe that Gandi is a real technology company and they believe they can offer premium support with support email. I agree, but I'd still like a telephone number for burning issues.
My only real complaint is that they have an unbelievably stupid login system where you log in not with a username, nor with an email address, but with an entirely random username. Mine is: PC5669-GANDI. It's infuriating if you ever need to use Gandi on a different computer. Why they saw it suitable to put their company name at the end of every login is puzzling.
Honestly though, its a minor criticism, and I love their "no bullshit" policy: http://www.gandi.net/no-bullshit
In my experience it is true.