The first game that comes to mind for me is Civilizations 4. I’ve probably spent hundreds of hours playing but after getting used to 5 and 6 I have a really hard time going back. Going back and forth between 5 and 6 I need to rethink some strategies but with 4 I feel like I need to rethink everything. I don’t know if it’s because of the tech tree layout in combination with the civics system, things like unit stacking, or maybe just a bunch of little things but it takes me a while to readjust.
LOTRO (Lord of the Rings Online) for me. Fantastic community back in the day, and the game itself was fun, too. Now, though, the gameplay is just too restrictive and oftentimes tedious by today’s standards. I really do miss the community, though. I made a lot of great memories there.