Glengara, you've hit the nail on the head! It is filtering and penalties that could be making the difference. I remember at the time I was reading both Polish and English language SEO forums I was very confused seeing how different the guidelines and techniques would be. Eventually, I've dropped reading Polish resources as they seemed to be 'lagging behind'. I believe the main reason for that is how likely it is to be penalized for spam in Google.pl comparing to Google.com or Google.ie.
English language results are the most important part of the business for Google, so it's obvious that the most efforts to improve the search quality goes to Google.com. Local Google versions in different languages may be a little bit neglected, therefore, it's much easier to use gray hat techniques. It's very common to use link exchange schemes in Poland, even SEO companies on the first page of Google use it... There's also many other 'crappy' SEO techniques they use and I don't think many of these pages get harmed by that.
I can imagine that 95% of Google employees hired to evaluate webpages and penalize spam would be working with English language pages and the other 5% with all the rest. If that was true, it would by much much easier to do shady SEO in Poland than in Ireland.
