I agree that Skyrim was polished... but that really crippled it's length and size, even Cyrodiil felt larger and Oblivion's main quest felt longer.
Uuuh... well; Oblivion had good expansions for instance too, Knights of the Nine was decent and Shivering Isles is probably one of the best expansions Bethesda has ever made (besides the unconquerable n#1, Fallout New Vegas' Old World Blues. The irony here is that this expansion was published by Bethesda but developed by Obsidian).
But then again, Bethesda seems to make better expansions than games.... except Dawnguard and Tribunal. The first for it's lack of originality (I can think of at least 3 better TES ideas for expansions than the overused vampire vs witchhunters cliché) and the second for it's limited scope and railroaded nature (Mournhold felt really small and the expansion can be completed quite fast. Also Almalexia is a joke of a final boss compared to Hircine's aspects)