I would say that the general guideline for me is that everything happens in the brain. I want to be able to believe that it's real.
As for the details:
1. Disciplinary, preferably parental spanking
2. Realistic setting and scenario (here I have some tolerance as long as it's not utterly preposterous)
3. VERY hard spankings, given without ceremonies, rituals, and too much blah blah blah before, NONE during the spanking.
4. Faster pace
5. Belt, switch, birch, electric cord, jumping rope
6. Some believable anger on the spanker's side - think "Semi-Transparent Mirror" or "For You I Will Come As Well" or even "Report Card" by RGE/Lupus
7. A girl young and cute enough to allow me to believe that it's indeed a parental scene
8. Severe marks, genuine reactions, tears
9. Natural and comfortable positions
10. The thighs being given their fair share
11. Consistency, staying within the role and no overtly sexual overtones
12. A really hard slap on her face and a few stray marks on maybe her calves or front side of legs is a bonus.