It's important to note that Freedom of Movement cannot itself impede movement:
From the PHB p. 233:
This spell enables you or a creature you touch to move and attack normally for the duration of the spell, even under the influence of magic that usually impedes movement....
So even if FoM itself had some magical effects that impede movement (such as some aspects of the frictionless effect discussed in this thread), the spell description explicitly states that the spell recipient can ignore those effects. Therefore, if it can swim, fly, burrow, or whatever without FoM, casting this spell on a creature cannot reduce these movement capabilities.
At the same time, the explanation that the spell has a strong frictionless component appears to be the best way to interpret the spell effects as far as explaining its capabilities and motivating house rules. FoM lies in the Abjuration school (in particular, not the Transmutation or Conjuration schools). It is much more likely that the spell creates a physical or magical barrier to things that influence or impede movement, rather than coating the recipient with grease, or reducing friction arbitrarily across its body or some such. (See the start of the Spell Descriptions section on page 172 of the PHB.) This best explains the way the spell helps a recipient ignore webs, fight effectively underwater, escape grapple checks, and ignore paralysis effects.
Most (if not all) of the difficulties with the frictionless interpretation expressed in this thread disappear when it is recognized that the spell is subjective with respect to the recipient (That is, the recipient has ongoing control over the spell's effects.), as stated in the spell description. The spell "
enables" (but does not require) the recipient to move and act normally, causes the subject to automatically succeed in resisting a grapple attempt against it or in escaping grapple (It does not require the recipient to attempt to escape a grapple or resist one if it doesn't wish to, for example, if a friend tries to catch someone with this spell who falls.), "
allows" (but does not require) the subject to move and attack normally while underwater.
This is very important, because the spell is long lasting (10 minutes/level) and cannot be dismissed. Harmless spells, by definition, do not have deadly or otherwise deleterious effects on their recipients, and there are no harmful side effects listed in the spell description. Creating dangerous side effects for spells that are designed to be balanced as defensive spells is very dangerous for the players (who may trust that a spell described as "harmless" is indeed safe) and can create play balance problems. For example, the instant-sink-to-the-bottom interpretation discussed in this thread makes FoM effectively a save-or-die spell against swimming creatures that do not have water breathing capabilities.
The ramifications of combining subjectivity with a strong frictionless effect coincide with the spell description and its apparent intent. A creature walking through a web spell chooses to be frictionless with respect to the sticky webs, but allows normal friction between the ground and its feet. The same creature swimming through water allows normal friction/fluid resistance against its hands/paddles/legs for propulsion and moves at it's normal speed relative to any water currents present. It can ignore strong water currents by being on the bottom and choosing to be frictionless with respect to the water, but not the solid surface. If grappled, the spell recipient chooses to become frictionless in the spots on its body where it is contacted, so that it slips out of its opponent's grasp.
A recipient of this spell that finds itself suddenly thrown overboard while at sea can choose to sink if it wishes (subject to normal buoyancy) or to swim normally using its normal swim speed or swim check as applicable. The spell does not enhance or otherwise increase any movement capabilities, it just allows them to operate unimpeded, so a cleric in full-plate will probably still sink like a rock (after failing a swim check).
Among other things, this means that a recipient of FoM can ignore hurricane force winds (but not debris carried by them) if standing on the ground. If not on the ground (or climbing on something or otherwise able to use another movement capability) the creature will be carried along just as any other object would be, modified by any fly speed it has. According to the rules for wind effects and Table 3-24 on p. 95 of the DMG, strong enough winds cause creatures to be "Checked: Creatures are unable to move forward against the force of the wind." This is clearly an effect that would be negated by the Freedom of Movement spell.