It would be nice to have metadata support in TypeScript. It can help the plugin of external information since comments for metadata is lame.
If the embedded information is made available in the AST, the extensible compiler (another feature request I saw) can use that information really well.
I can imagine the annotation in a similar fashion as .Net has it in form of Attributes.
If you like to make typescript an extensible language (I hope so), then adding attribute support can be a very good sample :-)))