I think GP meant it as a compliment, and a synonym of your phrase.

> Can't recommend it more.

"I'm physically incapable of recommending it more often/strongly because I'm using my full strength to recommend it already"

> Can't recommend it enough

"I'm physically incapable of recommending it as frequently/strongly as it deserves because I have exhausted my strength in so doing"

> Can't recommend it anymore

would be positive or negative?

Point taken. There is definitely some ambiguity about your phrase, and GGP's phrase, but in-context I think it was clearly a compliment. :)

"any more" is ambiguous, "more" is not.

yes.