That said, it is possible to go overboard and criticize something based on irrational hatred. At that point, you're just talking about trolls: it has nothing to do with developers not appreciating each others' work. Internet trolls have existed since the dawn of the internet, and there are plenty of resources on how to deal with them.
How you communicate your criticism is also important: there is no need to be overtly negative. Just point out what's wrong, and how to correct it; your job is done. Here's a recent instance where I've been overtly negative: the correct response is criticism of my style, as Junio has done [1].
[1]: http://thread.gmane.org/gmane.comp.version-control.git/22535...