[SimonPStevens]

I think it's about how much newness you have to handle at the same time. I can happily take a project using a framework or two I've never touched before. Or something in a new language but similar domain. But when you have the perfect storm of new language, tools and framework (and maybe OS) that's when it hurts. I'm not saying I totally couldn't switch, but it made me change me attitude a bit from '2 weeks and I'll be back at full productivity' to something more like '2 to the power of number of new things'. The more newness that has to be handled at the same time, the longer it takes to pick up all the interconnections.

[jghn]

This is also true, I would say it's a combo of the two. The more new things at once the more likely it is that you'll be working with something without a good correlation to something in your previous experience. But at the same time having a broader and richer base of experience will make it more likely that you'll have an easier go of it for any of those individual things.