IMO, the design differences in MM9 are at least half the fans' fault -- do you remember back before it came out when everyone was complaining about MM8 using such "old" graphics and being too much like the previous two games? (and also being too easy, incidentally.) The designers "fixed" those things in MM9 - different graphics, different storyline, and harder play. They got the play level right, as Peter2 says, but the rest ...

Unfortunately, the 3-D graphics system 3DO went with ended up looking dated almost before the game came out. But it is still a newer design than the previous 3 games. My own problem with playing it is that it gives me motion sickness to play too long (as does any modern 3-D first-person game).

Once you get past the differences, it is still Might and Magic, though. For instance, there is still a mainline quest to get multiple factions to agree and quests from each faction before they will agree and then more mainline quests resulting from their agreement. In this respect it resembles MM6 the most, probably, up to a certain point.

They got a lot of mileage out of using the new game engine to force you to do things disallowed in the old one (like swim, for instance). Unfortunately, the new system was not adaptable to flying, so that spell no longer exists; likewise, the wide open plains no longer exist and everything is channeled. That stylistic difference bothers a lot of people. The invisibility spell is gone too (most likely to make the game harder); however it was not in MM6 either. Wizard Eye is still there, but IMO fairly useless as the points no longer appear on a map, just floating in a display like a radar screen. Most other spells are unchanged or not changed too much.