[quote=@HeySeuss]
tl;dr: Critics should be humble as fuck.
[/quote]
Much agreed. Politeness and intent also matters significantly.

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I also think [@pugbutter] also is correct with offering praise alongside. Very rarely have I strayed from offering some form of praise along with it, but those were bios that were just so generic and thrown together that I couldn't really praise anything. But that was also when I was a GM and the bio just wasn't meeting standards.

Even in cases like above, I often like to point it out as opportunities to enhance the character, rather than outright criticism. It tends to work fairly well and most people I've done that with have been excited to do just that. Most of the people I've roleplayed want to do well, so I just try to empower them to do just that. The person from that scenario actually made a pretty good bio in the end - it is just sometimes, which I think also holds true for posts - people can get lost in their own thoughts, either from pressure of writing something they feel is sufficient, pressure from getting a post out quick enough, or just overeagerness. All things that can be easily guided in the instances it happens. 

Ultimately, it is a GMs responsibility to be providing commentary as they are the ones whose standards matter in the thread. You can offer advice as a player, but it generally should be advice someone solicited. If it isn't, you better be prepared to have it interpreted poorly unless you are some kind of criticism savant.