What is this plant??!!?

It's a Rubus species of some sort, most likely the common blackberry (Rubus fruticosus).

The reason you haven't seen any fruit is that blackberries fruit on last year's stems, and that stem grew this year, so will not fruit until next year. 

It can't be Rubus canadensis as another person suggested as that species is thornless, hence its common name smooth blackberry.

Rubus canadensis L., smooth blackberry

Wild rose/briar rose

That looks like a briar.