I've gone in the opposite direction: I've used Lisp for years, and now happen to be at a job where I do C#/ASP. So maybe the following is unhelpful, but if you have more specific questions I'd be happy to answer.
If you know Ruby, that's a big step towards Lisp already. Coming from C#, Lisp is going to be kind of like Ruby, but more so. :-)
Maybe it's because I was a Lisp developer long before .NET was even conceived, but I don't know of any particular pitfalls going the other way. There's very little I dislike about Lisp, especially compared to C#. Oh, if you're doing Common Lisp you probably owe it to yourself to get the ITERATE library instead of spending much time on LOOP (though you'll need to know a bit about LOOP to read other people's code).
If you're one of those .NET developers that uses Visual whatever-it's-called for editing, you're probably going to want to learn Emacs. You can of course use any editor to write Lisp code, but if you're not using something that's designed around Lisp's unique strengths, you're losing out on much of the benefit. SLIME is the most efficient development environment I've ever used on any platform for any language, though I'm sure commercial Lisp IDEs offer similar benefits.
You'll have "aha" moments from time to time, as anybody learning Lisp does. It's not for us to say what we think they'll be, and if we just started listing them, they wouldn't be "aha" moments for you, anyway. :-) But it helps to read a lot of code other people have written, because the difference in style and strategy between different Lisp programmers seems to be far greater than the difference in other languages. SICP and AMOP were the biggest eye-openers for me, but YMMV.
There are several web servers in Lisp. I've used Hunchentoot and Portable Aserve, but there are others. They tend to be fairly simple, and not full-blown "frameworks", since it's so easy to be productive without much boilerplate in Lisp, and since each Lisp program tends to be so unique.
I've got a public website built on Lisp, but I don't tend to advertise the fact, because (a) I do consider it a competitive advantage, and (b) I want people to say it's a cool website, without qualification. I've been told the "recommendations" section of Amazon product pages is also a Common Lisp web service these days, but of course externally it's impossible to tell.