This ramen is bland; the broth lacks depth.
His cooking is flat-tasting, so I wish it had a bit more delicacy.
Extended from the taste sense to describe performances, works, or personalities that lack subtlety, refinement, or interesting detail. Often implies something is too broad or simplistic.
That movie had a dull, unimaginative story, and I got bored quickly.
His acting is flat and lacks nuanced emotional expression.
Compound of 大 (large, broad) and 味 (taste). The figurative sense of 'dull, unimaginative' derives from the idea of something being too broad or lacking in fine detail.