If a writing mentor were to tell you "don't suck," what would you understand that to mean?
Well, considering one of my writing mentors actually told me that, I guess this is more than mere speculation.
When I got that advice, at first I felt it was like trying to nail Jell-O to a wall. It was too loose, too gooey, so formless that it didn't really mean anything at all. But without realizing it, those two annoying words had already done their damage and wormed their way into my mental operating system like a computer virus.
Each story I'd write after that, I'd find myself asking, "Is this the best I can do? Does it suck? Why does it suck? What do I need to do to make it not suck?" And so on...
So I guess for me, "Don't suck" was more a motivation than a technique. It was the underlying drive that made me examine my work to learn and recognize what sucked and what worked. In essence, by not meaning anything at all, it ended up meaning everything. "Don't suck" forced me to discover for myself what made writing effective and what made it defective.
So, in one sense, yes, it's like telling an apprentice to "Do better" without actually offering any practical advice on how to improve his or her technique and skills. Pointless. But in another sense, it's like dangling the possibility of being fired in front of an apprentice unless he or she takes the initiative to be responsible for his or her own career. Worth everything.