No, hes not going ethereal, hes passing into the spectoral dimension. The flow of time in the spectral dimension is slow (or is it fast?) enough to allow Raziel to do anything he wants in the spectreal realm without time passing in the material realm. If that makes sense.
So to put it in simple terms:
After Raziel disapears into Spectral realm, Kelthuzard will like "wtf" and perhaps less than a fraction of a second later, Raziel could have exited spectral and launched an attack on Kelth.
From Raziels point of view, Raziel can enter spectral realm...dance, sing, finish a game of chess and run a marathon before exiting the spectral realm back into the physical realm.
Then Kel'Thuzad might be in trouble. Kel'Thuzad is a highly powerful magician and the second most powerful in the Scourge ranks, but if there's one thing Warcraft as taught us, it's that its magicians are generally highly exposed if caught up close.
Force Raziel to run the whole way and he will go down, but Kel'Thuzad is not very durable. Get up close and you can bring him down with a well placed impale through his chains and chest.