In my opinion a scam coin is any coin with no new features or ideas (basic clone) that has a premine over 0.5% (for personal purposes) and IPO.
These coins serve no purpose other than bringing an unstable alternate currency.

New Features include:

- TOTALLY NEW Algorithm (Don't just add layers on top of x11)
- New way of earning coins e.g. Proof of Transaction
- New Design (At least the developers put some effort in)
- Additional addons to the Qt Wallet e.g. Block Explorer, Exchange, Games
- A coin built from scratch
- A coin with a powerful but possible idea. No examples for this one