Funny, I had the same conversation yesterday. A specific non-compete to say you can't work on the precise product segment (say, if you're at Facebook, you can't work on Google+ directly or something more specific like a segment your company is developing a patent in) for "x months" seems something that you can sign off if market segment is such.

If I pay for a car, I'm paying for a product. If I pay someone to clean my garden, I'm buying his time, expertise etc. for the time to clean my garden. I look at me being paid at work the same way. I'm paid to work for 'n' hrs a day, 'm' days a week per year for cost 'x'. I provide you service and I get paid in return. What I do after work, as long as I don't infringe on any IP from work, is my business.