Disclaimer: None of these are Jerkass genie questions and will not bite you in the rear for either decision (much).
1. A tank that stays in tank stance /w full fending the entire run or a healer that never touches cleric stance.
2. An Ice mage or a BRD/MCH that does not use WM/GB.
3. MNKs that only do DK-TS-SP and stay on the rear, or a DRG that only does full thrust combos
4. a BRD that only does straight shot, or a MCH that never uses reload.
Feel free to add your own.