The other choice is "threaded". Flat just shows the posts one after another in a long stream, page by page but threaded shows only the post you are currently looking at and the rest are in a hierarchical "thread"/outline sort of dealy below that where you have to choose each individual post you want to read. That has the advantage of having who posted it before you "get to it" so you can skip people you don't want to read or read people you enjoy reading first, etc.

Try it out; choose it in your Profile under "Edit Display". It's easy to change back and while it's kind of confusing at first (and thus kind of scary to me when I first saw it, what the ) one gets oriented quickly enough so you can see what's happening.