1. Do you know what "Delivery and acceptance" of the manuscript means and what time period should appeare in the contract as best for your book?
2. Do you know which subsidiary rights are most advantageous to keep and which to leave with the publisher?
3. Do you think mandated publication is a good idea?
4. Do you know the length of time that is common for the publisher you're querying to hold "reserves on returns?" Do you know how to change it?
5. Can you negotiate your own option clause so it benefits you more than the publisher?
If you aren't comfortable with all of these aspects of a publishing contract (or don't know what the heck I'm talking about!) then you need an agent.
Cathy