As a rule of thumb: if you can't find it in UESP, you're good to go. They can be related to the things you might find in a wiki, but I'm looking for the "human element" here.
I'm not referring to the generic positive aspects of the games or what makes them nostalgic (see my first comment for details.)
For example, traveling through the West Weald in Oblivion and finding a lonely little cabin all by itself and making it mine was pretty magickal.
Also, seeing the option to speak "Dragonling" in Daggerfall and developing an unhealthy obsession with them for the rest of the play-through was also magickal. *ahem*
So with that abstract intro, what makes TES magickal for you?