Okay. Non spoiler first thoughts.
Multiplayer : Horde is awesome.
But it is very, very different. I can't say that it's worse, but I'm not certain it's better. There are certainly parts that are better, but I played the original so much that anything DIFFERENT is still WRONG for me.
There's no more ranked/player match. Instead we have public and private matches. Ranking seems to be calculated in both, as far as I can tell. Public matches have dedicated servers, and are always full (Unless someone quits halfway through, and then they don't fill back up - as with Ranked in Gears1.) Private matches have a host, and don't need full teams (Public matches use matchmaking to fill up your teams), but they are also just that : Private. The only way to get in is through an invite, or by following a friend into them.
You go down much, much faster. All the guns seem more powerful, except the longshot which is no longer a one-shot active down.
The ability to crawl away when down changes things considerably.
The chainsaw duels are pointless, really. Any player with half a brain is going to shoot you instead, make you flinch, and then chainsaw you with impunity.
The flamethrower is cheap. Expect it to be nerfed in a patch.
Now, single player stuff.
The platformer elements in ActII seem as out of place here as the Kryll did in the first game. And the rail-shooter vehicle segments in ActI are as boring, yet frustrating, as the junker sequence in the first game.
You still have to listen to dialouge every single time you play through with no way to skip it, while your guy walks super slow. And it still makes you watch Locust take cover positions while you're stuck standing in the open, instead of letting you shoot them from the get go.
A simple tip : Shoot the giant *******s with shields in the feet.