Not all that good, well, etc.
Definition 1 of 2

Expectation Setting
Conveys slightly below-average quality, used to temper expectations in a subtle way.

Don't get too excited about the movie; it's not all that good.
Tactful Criticism
A gentle or polite way to express criticism without being too harsh.

Your essay isn't all that well-structured, but it has great ideas.
Conversational Tone
Common in casual conversation, not usually found in formal writing.

His drumming skills are not all that impressive.
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