I've also played through the first couple of missions. I was skeptical at first because the first boss is just a shrunken version of the MS5 lvl 1 tank boss and the second boss is from one of the old games too (forget which one). However, I am happy to say that the game is actually pretty fun and the animations are fluid- it looks to me like the slug anims are all intact. speaking of slugs- I am extremely pleased that you get the slug in both of the missions. It really irked me how little yo got to play the slug in the last few games!
The card collecting aspect of the game is weak so far, mostly because you have to beat a level without dying to keep the card, just like with POW's.
There is an overhead map that shows your progression and reveals the next mission once you have completed a stage. You can go back and replay any of the missions at any time as long as you have cleared it once.
I really like haveing a health meter, like you get when you are in a slug- I find it really enhances my attachment to the character and I try a lot harder not to die.
So even though I haven't played it a lot, Iam enjoying it and I can't wait to get home and play it some more!