I'll say what I said in the lead up to launch. Gears of War is a cover-based shooter, if you're sick of them, this game isn't for you but I think you'd be a bit ignorant to say it doesn't do anything new.
New characters, new enemies, new weapons, new mechanics, new game modes, new prodcedural weather system etc.
I also think the pace of the game has been tweaked with more destructible cover, more dynamic cover (there's a room in Act 4 that really pushes this idea) and a close-cover-combat system that really speeds the game up and encourages players to be more aggressive.
It plays better than any of the other games in coop with monsters like the pouncher and snatcher really demanding coordination.
Horde and MP have received an extensive re-tooling too.
It's still a cover-based shooter though and if you want something different, then maybe this game isn't for you but that doesn't mean it's not doing new things.
That's not to say it's perfect, but after 10 years, there's still not a single game that does what it does better. I want evolution but in a smart way and this is it so far.
Rod has already said to expect bigger changes in the next game.
As for how this stacks up against the rest of the series, I'd say it has a better campaign than Gears of War 3 and a better MP component than Gears of War 2.
Gears of War 4 feels pretty similar to the original but I think it has a better MP than the first game. However, I will say the former has some of my least favourite maps.
Impact is a camper's paradise and your spawn protection will run out just in time to get killed.