1.)If you post too often, I will likely unsubscribe. I like to be able to get through my entire feed-reader each day without leaving anything "unread", and it's hard to do that when ONE blog has posted 10, 20, or even 30 times.
Example: As much as I sometimes like Jezebel, they got axed from my Google Reader for precisely this reason.
Exceptions: The amazing Shakesville, which is stellar from post to post and has a perfect mix of commentary, anecdotes, YouTube fun, and open threads, and Queers United, which, despite posting several times a day, posts short and sweet commentary, news, and action alerts that I am sure to take the time to read.
2.) If your entire post does not appear in my feed-reader, I am unlikely to click over to read the rest of it. (In blogger, you can check or change this under the "Site Feed" tab in the "Settings" menu.) It's nothing personal, but unless I am so engrossed in your post from the title and first couple of sentences alone that I absolutely have to know what comes next, I am likely to just scroll on by it. And even if I DO click over to read it, I am annoyed that I had to open a new tab and wait for it to load.
In case you need more convincing to switch to displaying full posts in your feed, here's a scenario to think about. When I used to use my Google Reader as a way to allow me to read blog posts on a work computer in which nearly all blogs were blocked, having to click to a blog to read the post meant I had no way whatsoever to read it. I would sometimes try to mark the posts I wanted to come back to and read later at home, but it almost never happened.
What keeps you from reading blogs?