Excellent topic! I would venture to say that most compliments are, indeed, if not disingenuous, merely platitudes. Sincere, specific compliments are highly effective in boosting self-esteem and supporting positive behaviors (manipulating for good). It's also important to match tone to context (which you mentioned) and deliver with authenticity. The unfortunate thing about sincere, specific compliments is that they take work - much like giving a good gift. It's far easier to give cash or a gift card just like it's much easier to dish out non-specific positive feedback. Interestingly, in children raised primarily or exclusively with non-specific positive feedback, there is a significant correlation with the need for extrinsic motivation as an adult.