Dealing with promo emails is one thing I'll miss the most, especially pinning the 1 or 2 you might want to keep and then sweeping the rest with 1 click. On desktop gmail you can reproduce this functionality by starring messages you want to keep and then bulk selecting using the "unstarred" option.

The ios gmail app does not have any good answer to this, though. You have to select each email one by one and then delete/archive.