Latency is your worst enemy in WOW!
This page has some good samples of how lag might affect you in World of Warcraft. WOW has a pretty decent latency meter built into the game, since it is good at showing your average latency over a short period of time. This is helpful, but because it is an average, it is actually hiding the most important aspects of your game connection, namely when your connection sees large lag spikes and/or packet loss.
Typically when you see a large lag spike in WOW, your game will suddenly start reacting funny, you will miss moves, characters start to warp etc. You look at your latency meter and it shows green. WTF?!?!? Then a few seconds later, your in-game meter goes red, reading huge latency scores, and surprise, you are dead!
The great thing about WTFast is that it really helps to smooth out and stabilize your game connection. Lag spikes and packet loss is greatly diminished, dramatically improving your game experience, so the game does what you actually WANT it to do! WTFast literally makes a difference between a winning and losing move.
See below some great examples of just how lag can effect you in WOW.