Some answer could include something like:
- Data such as constants or client-specific logic never belong in SPs.
- Passwords or other security tokens never belong in SPs or any code.
- Complex processing may not be suited for today's rather primitive SP languages.
- Business logic may be better suited for the middle-tier rather than in SPs, due to differences in the expressive nature of the languages, maintainability, OO design, or other factors.
- Presentation layer code is probably best suited for the UI, not SPs.