Simple for me was getting and experiencing things i had not the possibility to do at the time.
In classic i managed to raid for almost all content (i did not do naxx40 due of focusing on Shadowlands release, big mistake)
In TBC i cleared all content plus getting Champion of the naruu/hand of A’dal and the amano War bear.
In Wotlk my main reason was getting the plagued and black proto drake.
Now for MoP I will just keep farming gold to get a spot as a buyer in GDK and getting the CE from the various raids.
Another object was setting up gold goals from one expansion to another.
In classic i managed to buy my epic mount and going into TBC with another 1k. That alone for me gave me a lot of satisfaction, when Vanilla was curruent in retail i was so bad that at the end i bought an account just to have a lv60 char with the epic mount. 