One of the weird things about WoW melee combat is that you almost always want a slow weapon. There's only a few situations where a faster weapon is better. This is because a lot of abilities depend on "weapon damage" and slower weapons have a higher damage range than faster weapons with the same DPS. So given two weapons, you almost always want the slower weapon.
This seems odd to me. It seems like there should be some advantage to using a faster weapon, some trade-off that would make faster weapons better in some situations. You'd think that effects that have a chance of occurring on a hit--also called procs--would be lend themselves to fast weapons, but because Blizzard uses a Proc-Per-Minute system for most of these effects, there's no advantage to a fast weapon.
(A quick explanation of Proc-Per-Minute (PPM): The probability of proc happening is independent of the weapon speed. For example, Seal of Command will proc an average of 7 times a minute, regardless of how fast or slow your weapon is.)
The few times a fast weapon is desired usually occurs when one of these two trends is broken. For example, combat rogues desire fast off-hands because none of their regular abilities rely on off-hand weapon damage, and because Combat Potency returns energy on every swing (a non-PPM system).
I think it would be better if fast weapons had some innate advantage, to balance the extra damage that slow weapons give to your abilities. Right now, I think weapon speed makes a bit too much of a difference in the quality of weapons, and that makes several drops less desireable than they should be.
Note: Slam Warriors and Hunters don't adhere to the slow weapon rule. Because of the way their abilities interact with the weapon swing/shot timer, there's usually a specific "best" speed for them. The weapons still suffer from the same problem, in that a weapon can be much better or worse than others of the same DPS, just because it happens to have a specific speed.