I already mentioned this in a thread on the 3DS forum, but I'd like to mention it again... it'll be short.
While I enjoyed Dark Moon, I was really disappointed. Luigi was way too much of a baby, I hated the fact that it was done in levels where you go back to your base after every mission, and I felt like Nintendo really Disney'd it up with the less appealing ghosts and overall atmosphere. Even the music was disappointing compared to that of the original. Dark Moon was a lot more forgettable unlike the original which had plenty of memorable moments, areas, and bosses.
Again, it just felt like a disappointing attempt to bring the Luigi's Mansion theme back. That's just me. It didn't need a sequel if you ask me.