I'm of the belief that designers should avoid leaving comments on a Dribbble shot such as "Nice!" or "Cool." For one, they serve no practical purpose other than boosting the recipient's ego. For two, there's the Like button that provides the same effect, with less clutter.
I think that, assuming designers want to help each other out, they should leave constructive feedback, either positive or negative. Simply highlighting what you like or didn't like about a shot will allow the recipient of the feedback to better themself. Moreover, it promotes the opportunity of discussion among designers, which is always beneficial.
Unfortunately, Dribbble doesn't enforce this kind of mentality. The only way it would work out is if individual Dribbble users state that as their "policy", just like Louie Mantia used to do. Perhaps the Dribbble community should embrace this modus operandi.
Disclaimer: I'm not a designer, but I follow many designers and I regularly browse Dribbble to gain a better appreciation of good design.