my biggest disappointment was in a game called Horizons. It actually came out, but without like 90% of the promised features in the game. I eventually did play it finally but quit in disgust very quickly. Originally the game was going to have like 30 or more playable races. And each race was like playing a different game almost. Goblins would need to band together in packs, cause they couldn't survive alone.
Dragons could fly and other players could ride on you. there would be merpeople, and the ocean was a whole other world you could explore with it's own cities and regions. and the merpeople and the land people could trade with each other at the shore. players could build and run their own towns. even having player mayors and kings even. and there would be gods who were mods that would log in and answer prayers to worshipers.
just some of the stuff they didn't end up doing. the game when it finally came out was just a bland Everquest clone