At one side this is great, all knowledge on this technology can be reused and the possibilities are infinite. On the other side: there is not a well defined boundary to do your programming in, where you are sure it works on all platforms. I think this is one of the advantages of a platform like Flash.
On the other hand: in a future release a micro version of the .Net framework will be embedded, maybe this will provide this boundary. In the mean time I forsee all the cross browser problems we all love in our current "old technology" web solutions.