Interesting post... I'll chip in with my 2 cents as well, and just concur with the previous poster here that Sweden is a really tough place to fit in. (And he was in Gothenburg, where people are much friendlier! I've lived in Gothenburg for over 5 years, and am now in my 7th year in the Stockholm area, and can attest to the very big difference between the east and west coasters.) And yes, the dark is a billion times worth than the cold, but also don't come to Sweden expecting to make friends for life or anything like that.
However, I would totally encourage you to come for a short period as I am quite sure that Sweden is a great place to come to as an exchange student. Generally speaking, the administrators here are super-super-efficient and very accomodating. Also, Swedish is relatively easy to learn (I have a close German friend of mine who studied in Uppsala for 1 semester and is *almost* fluent), and as long as you're speaking English (or American), most people are glad to help you out.
If you do plan on coming, I would highly recommend Karolinska in Stockholm. It's ranked very highly internationally and has an excellent reputation as a teaching hospital.