I don't think you can measure a casual player in Skyrim since the series isn't really a casual game. It can be played casually (and you'll just need to avoid whatever main storyline discussion pops up if you don't want spoilers) in the same way a casual game like Angry Birds or Candy Crush can be played hardcore by some people. Casual/hardcore is like a combination of time put into the game from the player's familiarity with the series/how familiar they need or should be with the series-end and the immersive nature, complexity, etc. from the game's end.
When it comes to MMORPGs it's all about the time put in. I like what SpaceTiger said about hardcore time and casual time. There are some casual MMORPGs out there that have very little lore and setting to explain the world and why it is the way it is (the company drops heavy lore and so on so that players can just jump in, you don't need to play regularly or hardcore to advance or make progress, and skills and stats are kept simple enough). We'll use something like AdventureQuest and Free Realms as an example of that.
Then you have hardcore MMORPGs that have a ton of lore behind them whether from literature, other games in the series, and so on. While casual players can play these MMORPGs, it's usually a steeper learning curve to get the gameplay mechanics, social elements, terminology/slang, and the lore is something that you actually end up being familiar with because it's deep. TESO, WOW, and EQ are examples of that. You're only going to get good with the time put into the game and you're only going to make significant advancement by...putting in the time.
I'd include LOTR Online in the hardcore MMORPG category. It's not all that steep a learning curve, but it has that you do need some time in the game to actually advance.
Most anime MMORPGs...I don't know where to put because they're F2P and thrive off of people buying this extra stuff to get a leg up. They vary across the board and really depend on what the gameplay approach is/what they're going for in the game.