Ask HN: Trademarking your Startup

I am in the final throws of launching my startup. A name has been chosen (which I have grown quite attached to), and now its time to address trademarking. Its a bootstrapped startup, using what time and money I can save from my day job, and $18k+ with single-class global registration is so far out of my league I was wondering:

1) Is a trademark important in this day and age? I already hold the domain - isn't that enough?

2) If i DONT have the trademark, could someone else register it and stop me trading under that name?

3) As a generic web application, what classes and communities should I be targeting to get good coverage for the least expense?

4) What sort of protection do other HN startups tend to go for when launching their products?