Learn Russian: Иногда просто делаешь искренний комплимент. - Sometimes you just pay an honest compliment.

This is a generalized-personal clause. By dropping ты, the sentence isn't about you specifically. Instead, it means people in general or one (as in Sometimes one just pays a compliment). If you added ты, it would sound like a specific observation about what the listener does.

In English we pay a compliment. Why does Russian use делаешь?

Russian uses the verb делать (to do/make) for compliments: делать комплимент. Direct translations of give (давать) or pay (платить) don't work for this collocation.

Why does искренний end in -ий here, and what case is it?

It is in the accusative case, modifying the masculine inanimate noun комплимент. Because искренний (sincere) has a soft stem, its dictionary form ends in -ий. For masculine inanimate objects, the accusative case is identical to the nominative, so the spelling doesn't change.

Why use the imperfective делаешь instead of the perfective сделаешь?

The imperfective делаешь pairs naturally with иногда (sometimes) to describe a repeating, habitual action. While perfective verbs can sometimes be used in generalized statements to show an occasional completed action, the imperfective is standard here for stating a regular, general occurrence.