You know, the kind of person who you tell stuff like "I really want to go to this thing" and they point out everything wrong with it.
Such a person might be a naysayer, "a person who habitually expresses negative or pessimistic views" (TFD)
A wet blanket:
a person who spoils other people's fun by failing to join in with or by disapproving of their activities.
a person who says something will not work or is not possible : a person who denies, refuses, or opposes something
There are always naysayers who say it can't be done.
Those naysayers who attack the integrity of the entire judicial system
A Debbie Downer, taken from the name of the title character of a series of noted Saturday Night Live sketches.
A [Stick in the Mud].1 It's a person who clings to the past or the way things have always been done and therefore not willing to listen to or try new ideas.