Pacing is vital in any game, including MMOs.
For one thing, an adequate pace keeps the player focused on gameplay and disallows them the leisure to pick apart every flaw of your game. You're unlikely to notice that clipping plane being slightly off while your attention is on the madman running at you with a bloody axe.
Of course, an unbalanced pace can be bad too. All but the most obsessive action-gamers need some small breather between battles. Slow points also can allow players to absorb their experiences, qualify them and encode them into memory.
Pacing plays any number of roles... but to what degree can pacing be controlled in a non-linear game, like an MMO? It seems to me that there are methods of soft control, like player-travel control through enemy placement and environment design. But are there any hard/certain measures of pacing control in games like these? What could be done to improve control?
I'm asking this with the belief that gamers frequently spoil their own fun, unknowingly most of the time, and that poor pacing is a key way in which that can happen. We shouldn't just leave it up to the players entirely.