<em>Why should a correction that has been made by a native speaker of that language get a minus?!</em>

Probably because it deserves one. <em></em>Unfortunately, there are sometimes some very poor corrections made on this site.

I won't name any names, but there are one or two non-native speakers of English here who call themselves 'professional English teachers', even though their own command of English is barely intermediate level. On more than one occasion I have come across notebook entries, 'corrected' by these so-called teachers, which are full of the most appalling basic mistakes.

<em></em>There are also many native speakers who have an imperfect knowledge of their own language. For example, in one instance, a learner had correctly written a sentence along the lines of 'When I'm tired I lie down on my bed', and a native English speaker incorrectly changed this to 'lay.' It is also extremely common for native speakers to write 'your' instead of 'you're' or 'it's' instead of 'its'. And I'm sorry to say that there is at least one native speaker professional teacher here who uses 'it's' incorrectly.

Added to this, many corrections are simply careless. What often happens is that correctors concentrate on a few errors and then miss others. If a corrector, especially a native speaker, fails to correct an error, then the OP will naturally assume that what they have written is correct.

If I come across a correction that is clearly substandard, especially if there are incorrect corrections and mistakes introduced that weren't there in the original, then I don't hesitate to vote it down. This isn't to upset or insult anyone. It's simply a signal to the OP that they should not trust what this person has told them. It doesn't seem fair to let them think that a text full of inaccurate grammar and misspellings is now 'corrected'.

Come on, Ocean. A thumb down option is a nice feature. At least it tells you how many people on the planet, acyually, want you alive :D

Ocean, I understand what you're saying, but I don't want to compromise my own anonymity in order to get rid of the bad people. It would be possible for someone very dedicated to find me in real life using the information available here on italki. If somebody got really mad that I voted them down, they could make my life really difficult, and maybe hurt me or someone I love. If people could see who voted them down, I wouldn't feel safe voting people down.

I don't have anything to contribute to the original question, but I have to say that I'm glad the thumbs up/thumbs down option exists.

In discussions, it's useful to be able to vote down mean comments, and vote up comments we agree with (so we don't have to type it all out when someone else already did).  There are some users here who are really mean, and insult every person who disagrees with them.  These people should be voted down, because it's a small way to show them that many people think their behavior is wrong.  And I don't want these people to be able to see that I contributed one of their downvotes, because then they will attack me!

In answers and notebooks, it's useful to be able to vote down bad answers and corrections, and vote up good ones.  Like Su.Ki said in the first comment on this discussion, some things really need to be voted down, so the language learners looking for help know not to rely on wrong information!

Of course, some people misuse the tool.  There will always be people who misuse a tool, no matter what it is.  What's important is to band together against immature people as a community.  If you see a good, kind comment, answer, or correction that was voted down, you can vote it back up!  And if you see something that is mean or wrong, you can vote it down.  It's important not to let a few bad people ruin a good site for all of us.

@ocean - It happens often, and I usually don't get too worked up about it. It's all in good fun. Maybe you aren't ready for the internet? Just sayin'...