They need to be hard enough that just stacking on levels won't let you cheeze past them without altering your strategy. A dragon is a dragon, no matter what level we reach. However, aside from being hard, they also need to be fair. Anytime you die to a dragon it should not be because the dragon is cheap, but because it is really hard and you messed up or underestimated it.
Basically what your saying is fights need to be based more on AI tactics than statistics that give it strengths and defences or health or spamming a powerful attack.
I'd both agree and disagree. I want better AI tactics of coarse. I was always disappointed in Fallout 3 things like radscorpians when damaged didn't retreat and burrow into the ground then when you stopped caring and started going your way went about a sneak attack.
Tactics and secondary behaviors basically. Skyrim's AI doesn't seem all that intelligent though. This goes for AI in games in general.
Where I'd disagree is that RPGs while about roleplaying is also mechanic and statistics derived. The lacking of mechanics in all BGS RPGs aside statistics should very well be a factor. Statistics in RPGs is always supposed to trump realism or belief. Statistics control the inexperianced player than they do it
I wouldn't worry on resource killing or difficulty though. TES games have always been on the more realistic side. This goes for Arena to Oblivion and probably Skyrim.
It won't take an hour like an MMO boss fight nor a Final Fantasy end boss. As much as Interplay, Ion, Looking Glass, Bioware, etc. Have been the anti-jRPG in this respect of bosses FF is more RPG than TES.
Taking about 50 potions of different sorts to boost and keep you alive while fighting a boss/end boss for 20-25 minutes is true to the RPG core.