Yes perhaps there it was kind of a mistake on Blizzard's side, but I'll say, a forgivable one since it didn't really matter to me much at the moment. Besides, they were equal then, remember? So they both just gave up. I think that Blizzard was trying to point to us their equal strength, with Illidan gaining favor over time but ultimately Arthas gaining the upper hand by merging with Ner'Zhul (before taking the armors, when NZ was channeling his power into him at the Frozen Throne) and as I mentioned before, the tired Illidan in snow, who actually feels and freezes, unlike Arthas. And tires. Arthas became even more undead after the "childish fight", in which he actually got tired since he was still human. And of course lets not forget undead > human, usually.
Now about Magtheridon, lets not forget Illidan was a demon hunter, granted the power of seeing all demons, being half demon himself (Gul'Dan), having gotten powers directly from Sargeras himself, the ultra god! Unlike Mag who was just a fag demon who probably never even met Sargeras. He was a pretty small pit lord, the winner, I think, was obvious in that fight. Oh, and draenei were able to hold so far, with some allies they could also probably fight back? And Naga? Op shit creatures, stronger than demons. Kael'Thas? His army was shit but he himself was pretty powerful. Vashj?
That is my argument about strength. About stealth, I think I can agree here, but probably draenei developed some kind of other stealth, don't put it in such gameplay mechanics. It isn't either he's stealthed or not. It's more complex, we just don't get to see it. There are so many ways to stealth, even in our life, imagine a world of magic. Draenei probably really could hide from demons, somehow, since that is what they had been training for, escape, survival. I am sure they had their ways. There are probably hundreds of types of magics of stealth and true sight doesn't work like in the game, you just see the transparent unit - it's probably something like increasing your smell instincts, or hearing ones, or seeing ghosts, etc. etc. Of course, I am not trying to defend the fact that they didn't explain it, but merely providing an explanation for those who need it, in that case, you, since you question that. You can accept that it is a mistake or try to bend it somehow to your appeal, to get more out of the product you're given. Unexplained things can be interpreted to your liking. In that case, I always choose in favor of a good product and just expand the idea of stealth in my mind which was never touched in Warcraft's lore.
Although I cannot interpret the fact that it was a frontal assault which pretty much sucked, but hey, at least I got something to defend the stealth theory. You could say I am unhappy from this lack of strategics in an RTS game, yes it could have been done better. I agree. There are many holes I do not like about Star Wars for example, but I love it so much that I somehow manage to ignore what wasn't as obvious and to mod some not well told things to my liking. What I get really pissed about are stuff like a crappy scene which I just cannot change. But stuff like - did Dooku actually have some ideological good ideas for the world and was just against the Republic for their crappy rule and laws n shit, but on the way of "fixing the world" he just crapped it up more? One can never know, it hasn't been truly explained. I can now choose - do I think of him as that character or the dull villain who just destroys for no purpose? Well, as I said, choose to your liking. It isn't like ignoring something that's a fact, that would be if I tried to somehow justify Palpatine's actions, since HE is the generic evil villain, one cannot argue here. Yea... so, I went deep into this to further my point on Illidan. His character was never truly revealed (ignoring WoW here) so I decided I can take things into my own hands and make him the character I always wanted him to be. It's not like I'm right, but neither am I wrong, right? Like, we argued about this - you think of him as an evil dude, I think of him as an antihero... but we both have no real facts to back our theories. Can we see good in his actions? Yes. Can we see evil in his actions? Yes. Can we be sure that he actually meant to do these evil actions? No. Can we be sure that he means to do good but just fails every time? No. Etc. etc. I talk too much, you probably got my point 200 sentences ago xD