What Bethesda basically wanted to do is take that "epic boss battle" and make it a regular thing. Problem is, humans are sentient beings so after the fourth dragon, they're no longer a challenge. Instead they become redundant and people get sick of killing them.
There's nothing wrong with the idea, it's just the execution is a bit off. In the base game Dragons are no where near strong enough or use enough of a variety of abilities for them to remain challenging and interesting for some players. A large number of players maxing out and in some cases exploiting and outright breaking the game with the crafting skills doesn't help matters. Try taking on one of them with out your Daedric sword of Uber L33tness sometime.
Still, a few mods to Dragon abilities, resistances, and AI patterns will liven them right up for you if you're on the PC which is what I've done. I almost never fast travel so with much stronger dragons, they become a moderately rare sight for me to come across and are always a difficult and random fight when I do battle one due to changes in their AI (they land a LOT less often) and use a wide selection of shouts in different situations. Not as much fun when an Ancient Dragon
Fus Ro Dah's you off of a steep cliffside as when you do it to some bandits, eh?