Rude office staff.
Wait time often in excess of 45mins
Every six months they have a new dental hygienist, which might suggest job dissatisfaction.
Twice I've paid my balance in full, at time of service, only later to receive a smaller bill for the same service. The latest was for $25...nominal enough that they know you can pay it and if you challenge them, they threaten "YOU WILL BE SENT TO COLLECTIONS AND CREDIT BUREAU." Yep, they write their letters and emails in all caps, complete with poor grammar and sentence structure. Note that the letter states they are going to send ME to collections, not my account. More threatening that way.

They lways have a screaming, unkept, unruly child in the building. The doctor accepts medicaid if that's a plus for you.