People *can* commit to other people's addons, but it's like the difference between "Can I?" and "May I?". Sure you can do it, but should you? and is it allowed?
Some authors don't mind, others do. The ones that do and got pissed off at people trying to commit to their stuff got lockdowns. Others completely pulled their mods from the svn. Sometimes it's translators that break an addon because they missed a comma or forgot to close a bracket. 100% of the time that this type of breakage is seen, it didn't have to happen.
I'd also like to pull this from the SVN Rules page that everyone has to *read* and *sign* when they get their svn account. If you want a lockdown for your project, that page tells you how to get one, too.
Do not commit to Addons that aren't yours
What am I talking about? You see a typo or quick mistake in another mod. You urge is to fix it or patch it. No big deal right? Well the problem comes from people doing much more drastic things to other people addons, like rewriting huge portions, or adding major features.
This is just a BAD idea. Don't Do It!
If you have something like this going on my first recommendation is to create a diff or patch file and send it to the respective author. Let them be the ones to decide if and how the patch gets implemented. If they ask you to commit the fix then that is one thing, never do it without permission!
In the past, we have had to revoke SVN rights of people doing mass commits to addons without asking their authors. We will do it again.
 THIS INCLUDES TOC INTERFACE VERSION NUMBER INCREASES!
If it's not your addon then do NOT bump the TOC after a patch. Even though an addon may seem to be working fine to you it doesn't mean it should be advertised as compatible with the latest version of WoW. There may be issues that could only be known by the author that you may not notice. Abandoned addons should keep their out of date TOCs so people can tell that they haven't been updated recently.
A problem I've seen is people breaking an addon that isn't actively maintained but is widely used. Lots of people get agitated, and noone who knows how to fix it cares enough to read the forums or whatever.
I can definitely recommend IRC for the starting developer. Don't be shy and ask questions, especially about SVN related stuff. People are interested in helping you. There are rough edges at times, but if one manages to not get hung up on them it's very good to be on there.
Another thing is that some information is really not put down in writing anywhere, but lives in some people's minds, and those minds often do sit on IRC.
Traffic on IRC goes from silence to extremely vibrant, but that's IRC.
IRC also serves as the commit-feedback space thanks to the CIA bot. People will be able to help you and also trace the steps with you as you do your first commits. SVN has its weirdnesses and some back support is never wrong.