1. Character customization/gender - The ability to play as a female and customize her is important to me, purely for personal reasons.
2. Akimbo guns - Visually looks way more interesting than just single-wielding guns. Any shooter with dualwielding easily ends up being played a lot for me and some of my favourite shooters ever have akimbo guns in them. It's often a deciding factor for how much I will enjoy a shooter, but not necessarily if other elements are good.
3. Gameplay variety - I love shooters that mix stealth, combat and exploration. Games like Deus Ex, Wolfenstein and Killzone: Mercenary being prime examples.
4. Graphics - Not necessarily the biggest deciding factor for me but first person shooters thrive on immersion and the more immersive it is graphically, the better.