It really was incredible. I am disappointed they did not finish the story arcs they set up for the main story, they had such great insidious plots brewing (Orchid?!) and in the end it was just a bomb at a convention. Human Revolution had a larger than life plot that only such a series can support, and they were almost getting there for MD as well but stopped short.
I ignored the game at launch and only had time to play recently. To me, it felt like a complete game given how fleshed out the sidequests were. The narrative seemed to be setting up for an absolute spike of a third game.
Admittedly, the other two deus ex games have stakes that ramp up low>medium>high through the game, but at the expense of sidequests and exploration. I figured the follow-up to MD would be able to hit the ground running and have plenty of space to be wildly high stakes the whole time. Maybe even get as wacky as the first did, but in a "plausibly serious" way.
I got both Deus Ex and Deus Ex Human Revolution recently on discount, and both games are amazing. My only complaint with Human Revolution is that it didn't feel as "deep" as the original story-wise, but it still felt leagues beyond other games.
I am going to play Mankind Divided, but everything I have heard is that Mankind Divided was disappointing so I am bracing myself.
I played the original Deus Ex back when it was new, also DX:HR and DX:MD. Of the two new ones, I find myself going back to replay DX:MD more. To me the biggest problem with MD was it's third act sort of fizzles out, but I enjoy the rest of it enough and the side quests enough to come back to it. Also, the really refined the gameplay to make it more fun.
I hope that was just setting up a sequel.