I grew up using British software. It would be very polite.

  Please enter your name.

Nowadays, I see this level of terseness in software:

  Name:

And error messages are usually an afterthought, so the concept of politeness doesn't even appear on the radar.

  Error: null.

There's a turnstyle I use everyday that simply orders you to "Leave" or "Enter".

I think UIs should apologise if something that's not the user's fault happens (no network connection); be polite when giving orders (please do this or that); and be very precise when indicating input errors and suggest a course of action.