Exactly my feeling and situation. After an MS in CS at a decent US University (my European BS is in Physics), I started working as an H1-b at a US firm. After two years, I am one of the most valuable members of my group and my employer definitely wants to keep me here in NY, however my fiancee is from Europe and cannot work legally here (even if I marry her). It is kind of harder for her to get an H1-b visa (her BA is in Classics, plus H1-b's are snatched instantly). The company lawyer told me that it is highly unlikely for me to get a Green card in the foreseeable future, no matter how indispensable my employer thinks I am, since according to the rules, I cannot use my experience in the current company as part of my qualifications to justify the Green card. And of course the fact that you might be exceptionally good does not matter in the application for a Green Card (unless you have made headlines - there is provision for Nobel price winners etc). So, I am kind of thinking of heading back home, of course I do make enough for a family here but my fiancee hates not being able to work and I can't blame her...
You don't have to say the US will loose if I myself leave (you don't know me to judge if it is the case), but I am sure there are many talented people out there in such a situation.