Contracts are most likely not your favorite part of freelance work. Hammering out a watertight, legally binding agreement can be a bit of a distraction when what you really need to do is get some work done for your clients. So your choices are: operate on a handshake, purchase an off-the-shelf contract, or employ a lawyer to draft a custom one.
I won't be back. (Termination) - An escape clause can be a touchy subject. Addressing the possibility that the project or working relationship won't work out can cast a dark cloud over an agreement, but it's necessary that both parties be protected.
oDesk's take: Hourly assignments may be terminated by buyer or provider at any time, and the Buyer is obligated to pay for hours worked prior to termination. Fixed Price Assignments may only be terminated by mutual agreement.
Who owns what? (Work Product and Buyer Deliverable) - Assigning ownership of Intellectual Property is important. Who own the source code? What happens if your client hires a third party to modify it?
oDesk's take: Providers do not retain Intellectual Property rights. Any copyrightable works prepared by Provider in connection with an Assignment for Buyer shall be "works for hire." Any materials that the buyer provides along with an assignment remain property of the buyer. The provider is obligated to destroy any copies of that material after an assignment is completed.
Don't borrow without asking. (Pre-existing Intellectual Property in Work Product) - Reinventing the wheel isn't necessary, but being upfront about what work you are building upon, is. Sure, you can build an e-commerce website at breakneck pace if you develop it based upon an off the shelf CMS, but does your client know about the licensing fee? What about the payment module for their merchant account, are you going to purchase it, or roll your own?
oDesk's take: If providers wish to use 3rd party software (or other IP) to complete an assignment, then they need written consent of the buyer. This includes open source software. It is assumed that providers will delivered unique IP that then becomes property of the buyer.
Be sure to also check out the oDesk Policies Page, it provides detail of the information given above, as well as some useful insights in to how to make the most of the oDesk Marketplace.