I've lived in London and in the US for a while and English is my third language (after Romanian and German). I'm quite proficient in English, not as good as my mother tongue, but I understand everything that is said to me and my vocabulary is quite extensive; however my accent is east-european with no doubt.

What I've found out is that while Americans are a lot more insensitive (tolerant?) to broken accents the British absolutely can't stand it. They avoid talking to people with bad accent and will generally walk away form any conversation with somebody of a non-british accent. I've observed this with brits around the world, not only in the UK and for sure it's not related to talking among conationals only: generally they are willing to engage in conversations with people that are obviously not british as long as they speak proper english with proper accent.

Of course you can not generalize the above to every brit in the world however this is my observation.

About blogs I observed in a comparative study that pages not ending in .com are just a bit less likely to be clicked.

About a written message, what can I say? If the English is badly broken I get annoyed too. If there are minor spelling mistakes it does not bother me, substance prevails over form.