Silverlight has several fundamental benefits. However, there's always a flip-side, and it has some shortcomings too. There are at least two major limits that I see:
Silverlight is still a very young technology. After the JS release, and a 2.0 alpha, beta1, and beta2, it still doesn't even have a combo box! However, I'd expect that the Microsoft eco-system will rush to fill in these gaps via open source and the Silverlight community. Silverlight is young, but I'd expect the Microsoft faithful and developer community will make it grow fast.
As a developer, I realize that Silverlight has its problems, and an uphill climb, but I'm optimistic. I think that soon its strengths will outweigh its weaknesses.