'Casualization' is least of my concern. Well, one of the least. Bethesda is getting big and to stay big, it must expand its market. Hence Skyrim is so hyped. It's good for Bethesda as they will profit from it. Of course, they must 'casualize' the game to bring in more client. They took out tedious elements like armor degradation and redesigned the character leveling (no stats, only perks). I think that was good because I didn't like Oblivion's system of 'armor degradation' (honestly, just clicking on hammers a few times, it added nothing to the game) and original stats system was a bit redundant and tedious. Those are just few good 'casualization' that I can agree with. Now, keep in mind that they could've reworked the armor degradation system instead of just taking it out and do the same for stats system. Personally, I would love to have those in the game and still feel important as an immersive element of the game but I don't mind their choice of design.
Now the casualization kinda feels shaky when they do stuff like, combining torso and leg armor. The arsenal of magic spells are... limited (no levitation, no silence, some magic spells are only available as dragon shouts like command animal) And the UI. Maybe you won't mind if you are on console but on PC, it's a huge turn-off. Of course, these can be fixed with a bit of modding and devs probably know that.
I think there should be other things to be considered. Skyrim lost some features but it also added more. Dragon shouts, dual wielding, smithing, perks, etc. I don't see why you would complain about the Skyrim world design as I think it is well done and beautiful. All parts of Skyrim are not the same. (forest of Falkreath, cliff ruined city of Markarth, snowy city on frozen river Windhelm) Dungeon designs are better in general, especially those big quest dungeons. The minor dungeons feels the same as it did in Oblivion but Skyrim is an open world game so you can't expect every dungeon to be completely unique and awesome. Anyhow, it is hard to see Skyrim as a mark of Bethesda going 'downhill'. It's an overall improvement from Oblivion and modding will only make it better.