This is the current definition of Free Movement as seen in the PHB:
Immunity to constriction, hold, paralyze, and slow Spells or effects, as well as a spider's webbing or similar sticky substances. It also negates underwater movement penalties.
This definition is so broad, it makes designing fun/interesting encounters very problematic. I can't tell you the number of times we've had an idea for a room that would have been fun to play, but we couldn't use because FM negates it. If FM was something only very few players had, this wouldn't be such a big deal. But since nearly all characters have FM, that fun challenge gets nerfed into oblivion.
We're considering making FM only negate magical movement restrictions, not mundane. FM would still negate the effects of hold, paralyze, slow, and web Spells. However, things like constriction/being grabbed by a large by a monster, non-magical webbing/sticky stuff, and movement penalties in natural fluids would still affect a character with FM.
Whaddya'll think?