I may be alone in this, but 'more puzzles, less story-line' is my reaction so far. Part of what I liked about the earlier games was the sense of wandering through the deserted realms, a lone explorer. I can't speak to MIII of course, but MIV has so far had a bunch of mostly irksome "interactions" with the author of the linking books and his annoyingly precocious daughter. They aren't really interactions so much as little mini-lectures, since you don't/can't answer back.
On the other hand, the environments are still gorgeous. I'll have to speak to the puzzles after I find some. Okay, some more - I've found several that come later but aren't currently active.