Firewall filters...

Hello there,
After 3 years of using mikrotik for wireless links only i decidet to switch my Slackware linux with RouterOS.
Can you give me a hint about what do you guys filter on your machines? Which ports and why?

Will appritiate any hints and tips.
Thanks in advance.

Please read wiki articles on firewalling, there are a lot of examples on what to filter and why
http://wiki.mikrotik.com/wiki/Firewall