Blogger

Advantages

- Comments Available
- Easy to change template
- Good for group blogs
- Archives

Disadvantages

- Blogger Navbar
- No password protection (even if you use javascript, it's very easy to hack)

Livejournal

Advantages

- Protection (Privacy locks, Friends-only locks)
- Comments Available
- Music/Mood Display
- Archives
- Good for communities and group blogs
- Displays friends' recent entries (very useful)
- User pictures
- Memories (most memorable entries are saved especially)
- Amusing memes (little quizzes on LJ that use your friends usernames to generate funny stuff)

Disadvantages

- Style overrides make it difficult to change layout

Pitas

Advantages

- Full control over templates (making it easy to integrate onto a website, which is a BIG plus)
- Archives

Disadvantages

- No comments available (but you can get from haloscan.com)
- No password protection

Diaryland

Advantages

- Protection (Journal lock, password protection)
- Guestbook Available
- Favourite entries (something like memories in LJ)
- Templates fairly easy to change (like pitas.com)
- Notes (where other users can leave notes for you)
- Archives

Disadvantages

- No comments available (but you can get from haloscan.com)

Xanga

Advantages

- Archives
- User pictures
- Custom skin walkthrough
- Comments Available
- Guestbook
- Protected listings (something like friends-only in LJ, but only up to 10)
- Blocked users (very useful if you have a flamer on your site)
- Posting Calendar

Disadvantages

- In my opinion, Xanga layouts are hard to change. But that's just my opinion.