I think a contract is super important. Without one you risk them giving you less than you expected and having no way to prove that they owe you certain services. I think you need a contract and you need to have it with you the day of your event. That way when it comes time to pay the bill, there is no question about whether they held up their end of the deal completely or whether you don't owe them the full amount because they did not deliver all that was promised.
I would ask to see a menu up front. Find out whether your guests can choose on the spot or if you need to tell them before hand who will be having what.
Find out how private your event will be. Will your party have a room or area of the restaurant to yourselves?
Also find out how many waiters/staff members will be dedicated to your party alone. Make sure it sounds like enough staff members or else you and your guests might end up dissatisfied with the level of service.
Find out about alcohol (if you want to have it at the reception). Is it included, is there a limit? If you bring your own, will there me a bottle opening fee?
Is decor an issue at all? Are they letting you choose the linens, etc?
Hmmm, that is all I can think of for now.