Po-mo largely has a reputation (not wholly undeserved) for being self-indulgent, wanky, pretentious gobbledygook where a bunch of fops and feminists pontificate of the gender based hermeneutics of Harry Potter. Discussing how Potter's wand, which fires out magic which is used to ward of evil, is a metaphor for the mystical power of the phallus, and the demonisation of the female who can only be subdued through male sexual dominance epitomised by the contemptuous act of ejaculation. And how this represents JK Rowling's intrinsic self loathing acquired through the Stockholm syndrome effect of constant oppression by masculine, hetero-centric cultural and sexual hegemony.
The difficulty with Po-Mo, as you mentioned, is what it actually constitutes. It incorporates things such as semiotics, the concept of the 'text', critical theory, (neo) Marxist philosophy, etc, which are important tools for making sense of many aspects of our experience. The excesses of po-mo come when it starts to deny that there is any such thing as 'truth' or 'reality', rather than the less extreme position of accepting that reality is a heavily mediated concept affected by numerous ideological and perceptual tendencies.
With deconstruction, it often resorts to a writer simply saying whatever they want about a text and it's hidden assumptions, rather than anything that has any objective justification beyond the writer's imagination.
There are aspects of po-mo that are useful, although arguably they are not unique to it depending on where you draw the boundaries. It does have a tendency to move beyond an acceptable application of these tools and techniques and descend into self-congratulatory vapidity though.
Overall, a mixed bag and best taken in moderation.