Paul & Ann-Marie -
I am feeling your pain! We were on this very same cruise and I have to agree with almost all of your comments!
This was our 3rd with Costa and we had always had great experiences previously so did not expect anything different on this cruise.
Cabin - Despite many complaints it never got below ~22/23c - the only person who cared was our cabin steward.
MDR - Very inconsistent in quality/taste/variety/service. The first night dinner took just under 3 hours and on average we were 2+ hours with the first dish coming to the table approx 1hr after sitting down. One night I ordered from the kids menu to find something I would be happy with (I am a picky eater, but can normally find something I can eat). Also Breakfast was always a Scrum, even though I thought they did a great Ham and Cheese Omlette, I could not handle the craziness.
Buffet - Very inconsistent in quality/taste/variety/opening. As you said it was a treasure hunt to put an actual meal together.
Teppanyaki - By far the worst we have ever been to. It was tasteless with some poor quality meats and the show was not good either.
Burger Place - Decent but nothing to write home about
PIzza Place - Yes you have to pay for the Pizza - This was by far our best meal on board.
Staff - Except for 3/4 people I found most staff to be rude/unfriendly. Now I don't expect a lot but common courtesy goes a long way.
Bars - Yes overall quite limited, but you should have had access to whatever you wanted with your package. We ended up in the Beer bar on deck 5. This would have been free for you but even at €2.50 a beer with our package was worth it. Also the service was top notch!
We did speak to the Hospitality Desk about some of our concerns and the International host called me back 2 days later to discuss them. Offered Pizza or Crepes as an apology, I suggested an OBC to cover part of the teppanyaki and she said she would see what they could do. Here I am at home and I am still waiting to hear. I want to be very clear, I was not expecting any credit when I complained, I really just wanted a colder room.
I did however think the Panettone left in the cabin on Christmas Eve was a nice touch.
I could continue, but overall we are just disappointed after great experiences previously.