Well no game is perfect. A game can get nothing but praise from critics and a lot of the general public, but still can't escape criticism. I've met people who can tell me that Arkham City, Skyrim, etc. was bad. It's as common as with negativly recieved games. A game can get totaly bashed by critics and the general public, but still has it's fans. I can name people who say Duke Nukem, Game Party, etc. was half-way decent.
To me, this just shows the way life goes. No one and nobody is perfect. Good soap may kill 99.9% of germs, but you'll still have 0.1% germs left behind. It's the order of things.
What I'm trying to say is that not everyone will enjoy the same games. I personally find Call of Duty the most overrated franchise on the market, but that's just my opinion. There's nothing wring with not liking Portal 2. As long as you have a good reason to dislike it, it's okay to have an opinion.