Just because one type of dragon is moderately challenging, does not absolve them from the mockery that all others before, are. Even knowing most of a dragon's routines should not trivialize the encounter to the extent that it can. There should always be an element of luck or chance that can throw off even perfect execution, lest all battles become a foregone conclusion.
There is a reason the aspiring swordmasters in Cyrodiil can no longer miss like their Morrowind counterparts
If you already know everything, then doing something is a trivial matter. You already know the AI behavior, and decided to exploit them, if you STILL don't win, then it's NOT a challenge: it's a mockery, "Hey, you can never beat this random jerk you nuts!", and thus a very bad design.
You would certainly be pleased with the sadism that you call "challenge", but those who want to
play and not be a master strategist do not need such luck. Is there even any point in preparation if everything will eventually fail due to some Godly intervention? Which can only be avoided by pure luck?
And I don't buy the casual argument at all. If people can tell us its too easy because our difficulty is not set high enough, we have every right to tell them it's too hard because your difficulty isn't low enough. It's better for things to be too hard than too easy in my opinion, It is easier to adjust downwards than upwards.
Some people grow with Super Mario Bros and Megaman (and Contra), and I respect that, but games these days are meant to be fun, to be pastime, not to be a dedicated job. Trying to make something uber hard only to cater to the most sadistic of gamers is not a good design. Difficulties should be an opt-in system, not an opt-out. First impression is very important, and so is self-confidence. Going with the "Hey, I'm pretty good at this, I'll try the harder guys" approach is better than "HOLY CHRIST THESE GUYS ARE F***ING NUTS, I'M OUTTA THIS THING" approach
Most certainly, you don't want the monsters in the first area to be Master Necromancers the first time you get out of Helgen