I had a hard time finding Diablo, because in the left menu I clicked Store, then clicked Browse and typed "Diablo". Zero results!

Store only opens the current promotion, and Browse only searches within the current promotion. It does not show results from GOG's entire catalog.

The only way to search/buy a GOG game that is not one of the 10 games currently being promoted, is to click More Offers, then searching.

This is very confusing for the shopper. A better UI would be:

Store -> renamed to Promotions
More Offers -> renamed to Browse Store.