Sorry but yes OP DID say complaint.
I think what this thread really boils down to is how each of us classifies two very specific things:
1. What a "nightmare" is.
2. What an approrpriate compensation is for said "nightmare".
I don't look at two days of extreme inconvience as a reson to classify my whole vacation as a nightmare. Granted I would have been really mad. I am lucky in the aspect that I have a DH who would very clearly, rationally and calmly be the voice of reason that we still had eight days left and let's make the best of it and enjoy what is left. (I had a Hard Rock Hotel employee steal my credit card and charge it up to maxed out while on vacation ~ talk about a nightmare).
2. The answer to this is different for everyone. If OP didn't like this they should simply have said no... that makes nothing up for me because X (I don't like the bus situation, I prefer a value, Instant FP would really better help make up for missed time whatever). To me....SSR would not be fantastic but DH he would be in hog heavan having a room that big instead of Pop. So even to our family it would satisfy one but not the other.
So OP can certainly (obviously) write letter if s/he so desires. Just make sure to be clear, consise and include the good and what exactly the expected outcome of the complaint is (i.e. make managment more aware, get room refund for two nights, use as case study ~ whatever just be specific so they will know what they CAN do that will be acceptable to close this ticket).
So to review response from our family to a move to 2 bedroom at SSR from Pop:
DH:

ME:

DD1 and DD2: