Just a few starters off the top of my head
- How much should be paid, and when (including the deposit)
- What coverage you will be providing on the wedding day (e.g. preparations, arrival, service, formals, socialising/reception candids, first dance, departure. what times should are you expected to be there?)
- Whether you're committed to providing a second shooter
- That you do not guarantee to get any particular shot
- When you will deliver the images to the client / how many images / what format (print/proof/album/JPEGs)
- Asserting copyright (are they allowed to make their own copies of the images)
- Client agrees your liabilities are limited to the amount paid
- You are not liable for e.g. missed photos due to interference from other guests
- How much do they have to pay if they cancel X amount of time before the wedding?
- How any disputes would be dealt with
I didn't bother paying for legal advice when drawing mine up, but it's very easy to find example contracts online to use for inspiration. If in doubt, though, legal advice before would be less stress than legal action later.