Some people have recently faulted others for commenting on only a small part of a piece -- whether a blog post, a newspaper article, a book, or what have you. But it seems that such narrow commentary is often perfectly apt.
Say that you read an article or a post, and find something in it that's mistaken. There are several reasons why you might not want to comment on the article or post as a whole, but only on the mistaken item:
You may agree with the rest of the piece, and have nothing much to add to it.
You may not know enough to have an informed opinion about the rest of the piece.
You may think that your criticisms of the rest of the piece would be banal, too long, or otherwise boring for you or your readers, while the criticism of the one particular item is helpful and interesting.
You may think that this particular mistake is emblematic of a broader kind of error, and thus use it in a post that's about this error.
In such situations, it seems to me quite proper to focus only on the one mistaken item. If you're right in your criticisms, then you've helped correct a mistake, even if only a small mistake. If you're wrong in those criticisms, then you should be faulted for being wrong, not for choosing to criticize a small part of a post.
The author of the original item may soundly point out that some criticism goes only to a tangential part of the item. The argument wouldn't be "X is wrong to focus on only a small part of my post"; it would be "X argues that this part of my post is wrong, but readers should recognize that even if X were right, this wouldn't undermine my key point." But that just means that the comment on the small portion is of limited utility -- not that such commentary on small portions is improper.