"Defeating a dreadlord in the mortal plane only sends it back to the Twisting Nether." - Eligor Dawnbringer
Balnazzar and Mal'Ganis have already demonstrated this. Other notable dreadlords (like Detheroc, Varimathras, Anetheron and Tichondrius) are also liable to return eventually.
Just to point out - there's a very good chance Tichondrius is dead for good. He was killed by Illidan after he consumed the Skull of Gul'dan and gained his demonic metamorphosis - by gameplay mechanics, he couldn't be harmed by anyone apart from demon-form Illidan. Considering he was one of the most powerful Dreadlords and we haven't heard anything from him since the end of vanilla WC3, I think it's safe to say he's gone.
Ontopic: I sort of agree that they've overused the idea of ressurecting dead characters. There's nothing wrong with bringing back some characters once in a while, and there will always be an in-universe explanation, but it's cheap writing. Death should mean something. It should be something that's not taken lightly and nullified by 'a wizard did it!'
There are some situations where a ressurection or 'non-death' is valid. If a return from death is setup beforehand (i.e. Kel'thuzad with his phlactery, or even Kael'thas - who as someone pointed out, technically never died), there's nothing wrong with it. It's only if the writers bring back someone with no foreshadowing that it feels cheap.
Basically, the fear of death needs to be more valid. Obviously there are a lot of possibilities to cheat death in a universe like WoW, with magic like necromancy and the in-universe concept of dead spirits and what not, but a line has to be drawn if it starts to feel like it cheapens the concept and power behind a character dying. Otherwise people will just go 'oh that character is dead, guess we'll be seeing them get ressurected in three expansions' time!'