Leave a lot, much, something, etc. to be desired
Definition 1 of 1
Pro Tip 1/3Negative Implication
The phrase suggests disappointment without directly insulting. It implies potential for improvement. 
The project results leave much to be desired, but I'm confident you can enhance them.
Pro Tip 2/3Common Contexts
Often used to criticize skills, services, products, or performances politely. 
The camera's image quality leaves a lot to be desired compared to the advertisement.
Pro Tip 3/3Subjective Opinion
Speakers use it to express personal dissatisfaction, not a universal truth. 
In my opinion, the play's costumes left something to be desired.