You see this sort of "bait and switch anger" all the time. Here's some examples of games I can think of off the top of my head (note I'm not saying any of these games were good or bad):
Dragon Age 2
Mass Effect 2
ArcaniA: A Gothic Tale
Ultima 8
Final Fantasy X
Civilization V
Fallout 3
The thing they all have in common is that the game(s) before them in the series were quite different and they all had a very passionate fan base. When the games on that list came out the fan base largely bought them without question because they expected it to be "like the first one only better." Some people then found out that what I loved about the previous games in the series was gone and they had a significantly different game. That doesn't mean they're bad games mind you. This isn't just limited to games it also happens in music and movies.
When a musician/band releases a new album that is a significant departure from their previous albums their old fans will often jump up and down screaming "sell out." They bought the album because they liked the previous albums and expected that if the previous albums were to their liking then the new album would be good too. I think the most drastic example I can think of is the Goo Goo Dolls. If you like their first 2 albums (garage punk/rock) odds are you hate the newest one (soft rock/advlt contemporary sort of stuff), unless you have a diverse taste in music.
Movies have the same thing happen. Every time an old classic is remade people kick and scream about it. Just look at the newest Star Trek movie. People who are not Star Trek fans generally like it. The hardcoe trekkies I know hate it with a passion. Although most say if it wasn't "Star Trek" they would like it. In other words, it's a good movie, but it's a terrible "Star Trek Movie."
TL;DR I don't think most people are saying that Skyrim is a "bad game" they are simply saying that Skyrim is a "bad TES game" or maybe just a "bad open-world RPG."