The article linked below seems like it was written for dentists. Every one of the six “Reasons a website can suck” are frighteningly common on dental web sites. A few years ago a dentist could get away with a sucky web site but those times have long gone. Today people expect a dentist to have a web site and if it is nonexistent or just crappy then they will simple click away and seek another dentist.
Read the whole article however three of the red flag no nos I see a lot are: Out of Date, Unprofessional and Sends the Wrong Message.
My favorite dental web site designers are TNT. They create a distinct site for every client and they will steer you clear of the six no nos.
Never, ever, tell someone their baby is ugly — or worse, their website.