Salt at the right time, and more than you think
Underseasoning is the second most common fault.
Vegetable dishes need more salt than an equivalent meat dish, because there is no naturally salty component and nothing that has been cured or aged. Home cooks familiar with meat cookery routinely underseason vegetables by a substantial margin and describe the result as bland without identifying the cause.
Timing matters as well as quantity. Salting vegetables early draws out moisture, which is unhelpful when browning and helpful when the aim is to soften them or to concentrate flavour. For roasting, oil and salt before cooking; for a quick sear, salt after the colour has formed.
Acid is the other half of seasoning and is more often missing than salt. A dish that tastes flat despite adequate salt usually needs lemon, vinegar or something pickled, and the effect of a small amount at the end is out of proportion to the quantity.
Tasting throughout rather than at the end is the actual skill. A dish adjusted three times during cooking is seasoned properly; one adjusted once at the end has salt sitting on top of it rather than through it.
The other seasoning that is chronically underused is heat, meaning chilli, pepper and the aromatics that carry it. Not to make food hot, but because a small amount lifts everything else in a way that is difficult to achieve otherwise. Most cuisines with a long vegetarian tradition use it constantly and at levels most Western home cooking does not approach.