You can make up a basis eg doctor was rude, too fast, not detailed enough, not compassionate, not empathetic, made you feel like and idiot, made you angry etc. The more you complain and disturb them the more they will give what you want. We are always told by the admin people that if people take the trouble to write in to complain it means that something went wrong and they were aggrieved enough to write in. Also write things like I want this doctor to be struck off from the SMC register, he or she is negligent and not fit to be a doctor. Things like that. Also cc the Health Minister to ensure prompt reply to you.
Only the fools don't complain. Trust me. Try it and you will find it very rewarding to complain. This is how Singapore works.
Not like in Canada, you complain at the store and kick a fuss start cursing they will call the cops on you and tell you to leave. Zero abuse tolerance policy.
In Singapore the more you curse and swear the more they get scared and will pay more attention to you.