It seems that compensation tends to be less competitive (at least in the non-supervisory roles). I think Fidelity recognizes that their name cache on your resume is "value enough" in their eyes for future opportunities outside of the the company.
Because Fidelity has so many seperately managed business units under the "Fidelity Investments" umbrella, the cultures can vary widely. I've interacted with some employees in other business units (outside of FMR) who seem to have a very relaxed work environment ("think I'll leave at 3 today...it's Friday, and it's so nice outside!") or generally seem to be less impressive on a talent level-- some business units' Directors or even VPs are less qualified/knowledgable than many of the non-managers in FMR.
That being said, there does seem to be either a little bit of arrogance from, or a little bit of jealousy toward, FMR in interactions between business units.