Combat. Oh god the fucking combat! How could they go from something so shitty in the first three games to something so good? At least for melee combat I am sure of this; never played as a flaming faggot. Anyway, this is definitely the biggest improvement, and a vital one too. Gothic games have a lot of combat, and they all turn into combat slogs in the end. Having good combat in Risen means that the ending combat section of the game is still moderately fun as opposed to "oh god please let this game end."
So what was so great about it? Well, basically the fact that there's a lot more that you can do. There are ten levels to each weapon skill, and each one gives you something that is important. There are your basic combos, but after that there are lateral blows, and charge-up attacks, and parries and counter-parries, and then more elaborate combos. In addition timing is very important in chaining combos; do it right and they'll be faster, but it's quite tricky. All of these things make combat interesting and more dynamic, meaning it rarely gets boring. In fact I'd say it's the best melee combat in an ARPG that I've encountered. In addition the enemies aren't retarded, and the humanoid ones have all these skills too and will use them on you. They will block, parry, and counter-attack frequently. And finally that retarded stunlock is gone! I can't begin to describe how much of a relief that is. In addition multiple enemies will always try to flank you and work well together, meaning you have to constantly keep repositioning yourself to keep them in front.
And no trash mobs! Yeah, I'm referring to the hordes and hordes of enemies that the previous Gothic games liked to throw at you in the final stages. That's pretty much gone. It's definitely quality over quantity here. In fact fighting more than three or four of the lizardmen enemies (which are your late game badasses) is pretty rare. So there's less of them, but as I said above, these guys are of the strictly non-fucking around variety. Much more enjoyable this way. There are a few instances where you do fight larger numbers, but in those cases you have a companion(s). It's really like night and day.