I think the other biggie is fear of failing, especially when what you want to be doing potentially has a high failure rate. I'm thinking of fiction writing here in particular - wanting to write a great novel, knowing how hard that work is, and being so afraid of failing at it that you procrastinate (and here I could just as easily say "I").

Just as you suggest two minutes, I use 200 words. "Just write 200 words and if it's not working and you still feel like you can't continue, then stop." Almost always, I write far more than 200.