When I was a teenager, I really wanted to play Halo with my dad. I thought he would love it but he refused to even look at it, claiming that it would make him nauseous. I just didn't get it, we played Mortal Kombat and Resident Evil with no issue, why was this any different? I told him it was in his head or that these newer games would cause less nausea since they looked and performed better. I couldn't convince him…
Only a handful of games I've played make me motion sick, but Half-Life 2 is really bad. I don't think I ever felt sick playing CS:GO, though, so it must be something specific to HL2 and not the whole Source engine.
Most recent game to make me sick was Metroid Prime, played in the PrimeHack Dolphin fork. I actually got full-on vertigo for possibly the first time in my life. When I went to bed that night the whole room was spinning when I got into bed. I don't drink alcohol, but stories of being way too drunk are what came to mind from that. Such a foreign feeling.
I bravely tried to play Metroid Prime again a day or two later. Increased the FoV as high as I could without causing glitches (there's a warning about the highest safe value), also tweaked horizontal camera speed, as I had felt like it wasn't turning fast enough and that maybe that was part of it. I think it was okay after that, but I'm still a bit scared of the game and haven't played it much.
The worst part of the motion sickness for me is that I seem to have to sleep it off, it doesn't go away on its own the same day, so as soon as it hits me, my day is genuinely ruined.