word or phrase for "positive comments" or "positive review"

I think that the word "kind" can be of help here. Kind comments... kind reviews etc., will be somewhat more subtle and may hint at the desired action on their part ;)

Hope that helps!

It's very difficult to come up with something that doesn't explicitly mean positive, but one way of doing so would be to use the word encouragement:

[Merriam-Webster]
: the act of encouraging : the state of being encouraged

encouraging
: giving hope or promise

In short:

The movie critic gave the actor encouragement.

While it's not made explicit if the comments are positive per se in terms of the actual performance, the overall tenor of the piece (and subtle implication) is that the critic sees, based on the performance (even if it was poor in context), positive things happening to the actor in the future.

We British do it all the time. One such phrase is not bad. Any

Ok, you bought all the papers: what are the reviews of your performance like?

Not too bad.

Any Brit will understand that this actually means “pretty good”