1. Graphics. Like you said, no brainer.
2. Campaign map. Such an improvement on that patchwork Risk campaign map in MTW.
3. I'm going to be accused of heresy here... but sometimes the AI is better, sometimes worse. One thing I don't miss from MTW is having to chase an outnumbered enemy round and round a big field before I corner it so it will fight.
4. Overall feel. I think it feels much more fluid than MTW. Maybe down to the campaign map, maybe down to the graphics, maybe down to god knows what. I can't put my finger on it but I just prefer the overall feel of M2TW.
5. M2TW has some great sounds too... "I'll bring the house down, eh Sire?" etc.
6. M2TW is better supported than MTW in respect of patches. Can't be bad.

My twopenneth.