How much 'table talk' is OK - and when?
I will take the other answers a step further and say that talking during the Night phase is simply not allowed.

The other answers give good reasons why it could be an issue, but here's what the rulebook says:


  At night, players who are not active (whose eyes
  are closed) may not move, point, or do anything
  to communicate with the active player(s).


Any form of talking can be seen as covered under "doing anything to communicate with the active player(s)". The rule itself is a bit strange, because no one except the active player knows who the active player is! So you can't avoid communicating with the active player specifically, you would have to avoid all communication.

Also, if read completely literally, the rule seems to imply that the active player IS allowed to communicate. But this is silly, as if the active player ever says anything out-loud, then they are giving away that they are the active player. It is safe to assume that the rule is intending to say that the active player is allowed to "act" by doing the things they are instructed to do.

Also, from the rulebook:


  After the night phase, players discuss amongst
  themselves who they believe the Werewolves are.


This makes it clear that the time for discussion is AFTER the night phase.

So I think talking about anything related to the game is strictly forbidden during the Night.

If you are talking about more just general socialization, then this is somewhat similar to other games... in a casual game, card games which strictly forbid "table talk" may still have a lot of general talk and socialization while playing, as long as nothing about the game itself is spoken. However, the nature of the limited time in which to perform actions, along with the importance of hearing the instructions being given, makes socialization during the Night phase a bad idea, even if it is not strictly forbidden.