OMG that's a good topic.
What's so strange with putting ketchup on your pizza. :P I always thought it's normal.
Heh, it's hard to think for me what is strange in Poland because I was raised there...
Maybe that:
1. Nobody cleans after dogs so imagine how lawns look like...
2. After plane lands people claps, at least they used to, this tradition seems to fade away... (it looked strange to even to me, are people happy that the plane did not cruch or what

)
3. Polish eat potatos with everything... Have a lot of names for potatos, depending on region... (ziemniaki, kartofle, pyry, bulwy, I am sure you have more those are the most popular)
4. In most schools if you have good marks you are "kujon" (what is English translation?) because of people jelousity... If you have bad marks you are cool... I could never understand that althought I am Polish.