None at all.
I've picked up the odd refurbished computer and printer over the years and never got a bad one. Ditto for my refurbished Kindle2.
If helps to understand that refurbs are *all* individually tested in the recertification process, unlike new gear that is generally *sampled* by lot.
Different companies have different procedures/guidelines but refurbs aren't always returned gear; sometimes they come from lots that fail quality assurance en-masse and then have to go through individualized testing.
http://en.wikipedia.org/wiki/Refurbishment_(electronics)