The main source of complaints is veterans to the series wanting skyrim to build apon what the series was known for and how they were created. When they found that Beth is taking an increasingly overt different path with TES
Now this is a sentiment I've read so often know. The treatement of Morrowind (and Daggerfall, if the poster is old enough) as RPG masterpieces, bravely combining Interplay/Bioware's MQ choice&concequence system and deep roleplaying mechanics.
Unforenately, the sad reality is that both games weren't fantastic stories with a gripping MQ. And the roleplaying mechanics were cumbersome at best, with most players having to worry which skills NOT to take/level in order to get decent ability score. The current system in Skyrim FINALLY does away with that train of thought, and although it still has its flaws it is an improvement.
I for one have largely ignored ALL MQs in every TES installment since Arena and I don't consider any MQ of previous TES games to be that gripping. Nor did the MQs have that much consequence for the overall story. You're always the same ol' prisoner who somehow dominates his environment and ends up saving everything, which in a subsequent installment gets footnoted in a book or two.
People who are bitterly disappointed are wishing for something that never excisted in any TES game. They basically want TES sandbox environment with Interplay/Bioware quality storyline and choice/consequence model.
Admittingly that would be awesome but the reality is, the two gametypes are also conflicting.
I consider Skyrim the best of breed and once the modding community really gets going, it will definitely be your € 50,- worth. It is now already in my opnion.