Wow. We all knew that they were secretly working on it, but it's a surprise to see it announced just out of the blue and, especially, to have it come out only 5 months from now. Usually, Fall releases are announced at least a few months earlier.
Was Civilization V really 2010? That was so long ago, yet it doesn't feel like it's been out that long. My memories are still pretty clear about discussing it here when it came out.
I'm excited for the city tile improvements. They really sound like they'll shake things up and make games more unique.
I'm not wild about the slightly stylized graphics, either, but they could be worse. Also, early Civ screenshots tend to be slightly disappointing, since they're zoomed in too far. When you actually play the game, and at the normal zoom levels, the details tend to matter less.
That being said, I'm not sure how they can build upon and expand from Civ V. I mean, after Beyond Earth, it had everything. Sure, there are a few obscure and hyper-specific civilizations they could add, and maybe a bit more complexity to things like diplomacy and government/social policy - but really, nothing that is going to break the bank and turn the game on its head.
Have you read the polygon.com preview that was linked above? Some of the changes do sound like they'll greatly change the game for the better.
Besides, a lot of people had the same concern with Civ V. Most considered Civ IV to be perfect and wondered how they could possibly improve on it. As it turned out, though, moving to a hex grid and unstacking armies really changed the game for the better. It's actually hard to go back to Civ IV after getting used to Civ V. It sounds like Firaxis has some things in Civ VI that'll similarly change the game for the better and make it hard to go back to Civ V. Maybe in the the past, I would've complained about the developers changing things in one of my favorite series, but I've come to really trust Firaxis. They always hit everything out of the park and never disappoint, even when updating absolutely beloved series (like Civ, XCOM and Pirates!).