I find the notion of sharing libraries between the client and server kind of odd, in that people who talk about the "open web" think it's a desirable quality.
it allows you to rev a protocol faster, but at the cost of not being forced to have a point of reference that isn't also intrinsically tied to one implementation of the protocol, which seems a very un-open thing to do to me.
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Things like form validation, user input normalization, navigation options. There are ton of small data manipulation functions that you either have to maintain in both javascript and your server side language and cause some weird bugs when one version doesn't behave exactly the same as the other or else you need to make a bunch of unnecessary ajax calls just to avoid reimplementing things in js.
There is much deeper integration you could do but I think we're on the same page as to why that's a bad idea.