I haven't played BE why does it blow?
There's no real feel of progression, the tech web idea seems good, but because none of them tie in with historic milestones, you have no feel for progress. Civilization in many ways ties you in because your leading your people to greatness, every tech, every city adds to the story, its a race against other civilizations. And that's the other bit.... Civ doesn't need to put much story behind Napoleon. When Ghandi launches nukes at you.... the irony isn't lost.
But the BE folks, they don't pull you in enough for you to really give a fuck.
One good thing BE did, and I hope Civ 6 incorporates. Largely your army units never became obsolete, you just gained 5-6 more types. And new tech just let you choose upgrades for them.
Having to go pay gold to upgrade each and every unit when you get new tech in Civ V was a hassle.
The other bit I hope is they reduce and make more meaningful the buildings in your city, given that they are now being tied to landscape, it seems that way.
Edit Here's an article with the details: http://www.pcgamer.com/civilization-6-everything-you-need-to-know/