In general, the sequel feels more polished than the original, smoother and more intuitive to play, and it also feels much more "modern" in its mechanics, despite being released only 2 years later. Whether you consider that a good thing or not depends on your taste. For example, levels are a little smaller, less labyrinthine, and simpler to navigate, and there is a compass on the HUD with objective markers. Some of the streamlined elements are undeniable improvements though, such as being able to review your objectives at any time, being able to scroll through your weapons & gadgets with the mouse wheel, and the skill point system, which properly rewards the player for gathering intelligence and gives them an incentive to do so. The visuals were also drastically improved and have aged so much better than the original, keeping with the vibrant and slightly cartoony look but incorporating a much higher level of detail and much better animations, particularly noticeable in the cutscenes, which are so much more enjoyable to watch here. In many ways this one is still aesthetically pleasing in 2018, and I can't really say the same about the original NOLF, apart from its art style.
The stealth mechanics are much better too, and stealth in general is more entertaining. There's a proper crouch toggle now (which is invaluable), you have the ability to lean around corners and carry bodies, and there are visibility notifications in the HUD. Stealth is no more forgiving otherwise though, so quicksave is still your friend. The world is so much more interactive; you can open shelves and cupboards to look for items and intel, and turn off lights to hide yourself from guards, and both things were very welcome additions.
There's really only a few things I can complain about, gameplay wise. Firstly, enemies respawn in this one, which keeps stealth a constant focus, and isn't done in a very grating way; BUT it does mean that any sense of relief you're granted by methodically removing enemies from the level is only temporary. Another minor problem of mine is that grenades and other throwables are really weak now, making them almost never worth using. The snowmobile is back again, driveable in one mission, and sadly still controls like arse.
But it's NOLF again, with everything that made the first game so entrancing, but even better. An excellent continuation of the story, some really creative and sometimes ridiculous situations, great humour, fun new gadgets and weapons, and fabulous music. What a shame we'll probably never see a third game, and it's a true injustice that this series has been so easily forgotten. The sequel in particular is a wonderful achievement that everyone must play.