DX2 and HL2 are closer in terms of design and gameplay than DX and HL, from the way it sounds. HL2 seems to be taking more to DX's "solve things your own way" approach, and DX2 is taking out the skill system and replacing it with a deeper augmentation system. The difference is in that HL2 revolves around the static character Gordon Freeman, while DX2 allows you to play a character of your own design.
And anyone who thought Deus Ex sucked is wrong. Anyone who knows me knows how much of a DX fanboy I am, and that there is no possible circumstance in which I would back down on this statement. So don't bother replying.