Why not opt-out? If it's a good feature, we should
take the step of
giving it to people by default. If it's too intrusive, perhaps we can
finetune it?
I guess the intrusiveness will be the least problem. If you add this
feature to Wikipedia per default, Wikipedia servers would have to be
doubled, at least (each such preview counts on the server side like
accessing a complete article).
Sören