Crwth pointed out this post by Mike Mearls and while it's somewhat old news (June 17th!), it's worth commenting on.
First and foremost, dragons (or at least the one in the link in the bottom of the post) look to be pretty damn formidable.
For the first time in forever, if ever, the dragon strikes me as something that should not be fucked with. At least not without the characters doing some serious homework and planning. So for that alone, I say yes to the "legendary creatures" concept.
We've had parties where the characters were still in the middle to high single digit levels but kicked the ever loving crap out of black dragons; without breaking a sweat. Yet, the one Mearls details (albeit a preliminary and far from written in stone version) seriously looks like it would give a well armed party of 10th levels a hard fight. Even more so if they stumbled on it's lair.
Speaking of the lair, I love the extra things that happen just from being on the legendary creature's home turf. Getting buffeted by water or having the lights go out for a turn will throw an already hectic battle into total chaos. I love that! Because it's that sort of thing that makes an encounter memorable.
In the thousands of monsters we've killed over the last decade, there are just a few that stand out, and it's because they were extra challenging yet our party rose up and overcame. Or someone did something inane or stupid enough that we can all laugh about even years later. Like telling a party member to go ahead and fire that lightning bolt through me to get the bad guys behind, because "no worries! I'll make my save."
Anyways, as nonplussed as I am about the subclasses and circles and paths and crap, I feel the exact opposite about the legendary creatures. They've honestly renewed my wavering faith in 5E.
Now watch... they'll squash it in the next playtest packet.