I know that feeling well: it's what makes me hesitant from time to time to learn new technology. I think about all of the domain experience I would immediately not have. When I first started I didn't know what I didn't know and thus just dove right in.
I keep wanting to learn Ruby/Rails (and I have been working my way through the Rails Tutorial book online, albeit with a break the last week or so for travel and stuff). At the same time, I keep thinking "You know, I have so much domain experience with C# and ASP.NET, so would it be a better use of my time to focus on learning ASP.NET MVC and improving my architectural skills?"
I'm still working through that, but in the mean time, I'm pushing myself to at least go through the Rails tutorial (and probably an all-day Rails session at the CodeMash precompiler day) on the grounds that it's probably a good thing for me to break out of my narrow MS tech focus and gain exposure to other ways of doing things.
I think the only real constant in this field is that if you're not continually learning something, you're falling behind.
I think it is very important to learn a few different technologies from time to time - they often expose you to various concepts you haven't heard of before. My personal experience with Rails (reading the source code itself and that of its plugins) was that I got so many ideas how to do stuff that transfered quite well to my other projects.
It happens so often that after I switch technologies, and then return after awhile I have a "wow it is so much easier to do it this new way" feel. Creative thinking is fundamentally a "combine and use old stuff in a new way", which means that learning new technologies is worth it just because of the new ideas, and if you can later use those technologies themselves - well thats just a bonus.
Anyway, the original comment is right on the money. I often have to tell miself "just write the damn stuff and refine it later" when I get mentally stuck thinking about ways in which it could break.
