It could be a variety of things for me. For one, we just got Dawn of Sorrow a year ago. Now, I'm not condemning the idea of getting a Castlevania every year, but since this doesn't change much, it really does just feel like an extension of DOS, with a couple of new characters and levels, while very nice, cool, and with great music, just don't really do much for me. I liked going outside of the castle but I just don't think they did a very good job of it. Plus it did get a little repetitive after a certain.. uh point which I won't bother to detail. Oh, and the actual castle itself this time is extremely bland and poorly designed.
Another thing is that I'm more a fan of the old school, level based, please shoot me difficulty of the older installments. Castlevania IV is still my favorite, followed by Symphony and then probably Dawn of Sorrow. But Portrait of Ruin IS a good game, I had fun with it, it's just that I guess they didn't change things up enough after only one year or so of almost the same thing. I love the formula, don't get me wrong, but there is a way to keep the formula fresh and I'm not under the belief that they accomplished that with this game. I mean when I got Dawn of Sorrow, it wasn't too long after I played Aria of Sorrow and Circle of the Moon, and I still enjoyed it so much that I'd say it's my third or fourth favorite installment overall as I pointed out above.
Still, like I said, Portrait is a very good game, IMO. I guess I just expected more from it after they said it was so much bigger than Dawn of Sorrow... I put 30 hours into Dawn of Sorrow getting everything, and while I haven't gotten everything in Portrait yet, I beat it around the 8 hour mark (which IS good for a game like this) but I think I'll be tapped out by 15 hours. Oh, and the game is far too easy. Sorry, it just is. I was hearing word that it was the hardest Metroidvania and apart from a COUPLE of hair-raising boss fights (which you come to expect anyway), NO, this game is not difficult whatsoever.
As of right now I'd give it a 7.5/10, but that may go up after I've explored more of the secret/hidden stuff.