If the word "Casual" had any meaning, then Skyrim would be its definition. Skyrim is no commitment, you can just go anywhere, do some stuff, feel rewarded, drop it for a week, and come back. Sure, you may have forgotten stuff, but it doesn't matter. You can enjoy it fine, you missed nothing.
And that's good! That's what a sandbox game like this is for. You can play it hardcore, just like any other game, but looking at what the game purely is, then it's the most casual friendly RPG on the market.
Casual isn't bad, or rather, it shouldn't be bad. Sadly, dudebro gamers, as usual, pretend it's bad, just like they pretend blue sky or colors are bad, but that's dudebros for you.