Mediocre service:
Expect that they mess up your order 2-5 times per visit. Service is slow, except when they judge that you're about done, then they will rush to get you out.
Mediocre quality sushi, mediocre quality deep fried goods, low quality cooked dishes, abysmal quality soup or dry noodle and rice dishes.
Expect decent (salmon, eel) to almost inedible, rancid raw fish ("tuna", "snapper") . Better stick to cooked or deep fried stuff if you have weak stomach.