Well, here are a few issues that I had had with RM2 (, and was disappointed with RMSS when it came out) that are addressed in RMC:
1) Optional rules and core rules are better signified.
2) Power creep (I freely admit that I was part of the problem as one of the authors of the Doppelgaenger. Hey, I also wrote the Shadow Mage and it was more balanced) is now under control.
3) Similar to number one, but it appears that in future companions rule systems addressing the similar rules issues (e.g. spell research, magic ritual, to name just two) will be presented as optional.
4) The organization of RMC is much, much better.
5) The production quality is better as well. Not that it was substandard then just it is easier and cheaper to make better products to day.
Those are just a few improvements that I notice. There are mechanical tweaks as well, but it has been years since I played RM2 and I feel the changes just cann’t put my figure on them.
All of that said it would be wonderful if we could get a comprehensive list of changes made, and an general idea of how compatible each companion now is.
All of that said. I really like taking everything back to basics. I am teaching a new group RMC and it is much easier than back in the day with seven companions, SUC, house rules, etc…. I must admit to that one of my players back in the early 90’s was always driving up the power level of my game s and now I am a little more free to conduct a lower powered game.