I guess most English native speakers get more requests than they can handle. If you are serious about the language exchange, both partners have to be a good fit, otherwise the exchange isn't effective. Also, you can only have a good and intensive language exchange with a very limited number of partners. After all, we also have lives outside of language learning.
As a German native speaker I also get a lot of requests for German and English. Actually I disabled the function "add as a friend" and I clearly state in my profile that I am NOT looking for a language exchange partner and that I am NOT available for Skype exchange, but a lot of users ignore this and send me this typical "Hi"-messages anyway. So, I second what Chris wrote. Especially number 1, 3 and 5.
So, sometimes, I ignore these kind of messages (especially if the profile of the other user looks fishy, i.e. maybe fake profile, only interested in dating etc.). But if it is a new user, who seems to be serious about language learning, but just doesn't seem to know how to effectively approach potential language exchange partners, I send a reply and explain that I am not available for regular language exchange, but that I am always willing to answer questions about German. And I add the link to this very useful discussion by Richard: http://classic.italki.com/discussion/110575, with helpful advice for English learners on how to find language exchange partners.
1. They don't read my profile where I state I'm NOT looking for a language partner.
2. They don't speak any of the languages I'm learning or wanting to practice.
3. They write a short one-word or one-sentence message like "Hi".
4. They want someone to teach them a language from scratch without taking any professional lessons.
5. On my end, I don't have a regular time to have a language partner.
1. They don't read my profile and know nothing about me whatsoever
2. They ignore the fact that I am not looking for a language partner.
3. They think that italki is a dating site
4. They are not really looking to learn a language
5. Their profile is empty
6. They think I will do all their homework for them for free.