1. I can listen to them on my iPhone while I am traveling.
2. They load fast and get right to the point with no drama.
Myspace and Facebook are my least favorite. I can't listen to them on my iPhone (no flash) and they usually take a long ass time to load the page (myspace in particular) and facebook likes to make me log in and 'like' a song before I can even listen to it which is the kiss of death for you. I will not take the time to log into Facebook just to listen to your song let alone put my reputation on the line by 'liking' a song that I have never heard. Especially when I have thousands of other emails with demo links in them that are not a pain in ass to listen to. The easier you make it for me to listen, the better chance you have of getting listened to.
Reverbnation is ok, but not preferred. I can't listen to reverbnation on my iPhone.
If you're including an audio file in your email, don't send me anything but an MP3 or I will not listen to it, most likely cause I can't.
Sendspace and other file sharing services that I have to download to my hard drive first are the WORST mainly because they expire after a certain amount of time and I hate having to take up my hard drive space. Don't send me these. Lately I have just been deleting these emails without listening.
Just a litte FYI for you. Keep it in mind when sending me your demo.