To be honest, I've probably accounted for many of those downvotes without comment in the
[iphone] tag. At first, I left comments for every one of the questions I voted down, in an effort to try to help people write better questions. However, as of late there have been more and more bad questions being written and I just don't have the energy to leave a comment for every single question clarifying what's wrong with them. I found that very few of the people learned anything from the remarks I left. Some of the askers even responded negatively to them.
In cases where there is a good question, but poor phrasing or broken English, I edit the question itself to clean it up. We don't want to discourage those who actually try to use the site correctly, but aren't proficient in English. This is why I like the idea of flagging questions that just need a little editing.
I try to be polite and patient with people, but there are so many duplicated questions that could have been found with simple searches, "do my work for me" questions, "what is wrong with this code that is only applicable to my particular case" ones, and just plain unintelligible questions that sometimes I simply choose to vote them down and quickly move on.
Now that downvotes are used as a metric to help block undesirable users, we even have an incentive to vote down just plain bad questions to prevent users from abusing the system. Negative votes can also motivate people to close and / or delete these bad questions so that we can remove them from the system.
The popularity of the iPhone and iPad, combined with how helpful the Stack Overflow community is, have driven a lot of help vampires here (as Shog9 notes), along with people looking to get rich quick writing iPhone applications without putting in any effort. As SO appears at the top of more and more Google searches for programming problems, this will only get worse.