Short answer: the most common mistakes are unclear offers, weak proof, poor photo choices, hidden contact, neglected mobile, pages without search intent and no post-launch follow-up.
Being decorative before being clear
A website can look pleasant and still fail. If visitors do not understand what you do, for whom, where and how to contact you, design will not fix it.
Hiding proof
A small activity sells trust. Proof should be visible: work, details, reviews, place, process, finished projects or examples.