You'd definitely want to know how much standing inventory there is and how long the homes have been listed. The longer homes are on the market sometimes, the more flexible the builder is with giving incentives to move the properties. Often, they can't begin construction on the new homes until some of the inventory is partially depleted. You'd want to know the estimated completion date (but don't bank on it--new construction typically has delays unless the home is already complete), what your standard features and options are, whether you have a pre-construction meeting to pick out design choices, what the incentives are for using their preferred lender or attorney (at least stipulate in the offer/contract that you want to have your own lender look over the Good Faith Estimate to make sure there aren't any fees sneaking in on the back side), what the estimated build out date is for the neighborhood (when will they be done), what will the allowable percentage of rental properties be (can affect financing with resales if the % is too high, according to some lending guideline), when will the HOA be taking over for the builder/developer, HOA dues, what the dues cover, are there any capital improvement assessments due or pending, what's the builder and structural warranty like, etc. There are lots of things to anticipate with new construction.
While the onsite agent is required by law to work fairly and honestly with the buyer, they represent the builder...and getting both sides of the commission could mean that they won't necessarily negotiate a good deal for you like a buyer's agent would. The seller pays the commission in this market (most likely), so if having a buyer's agent is free, you may as well have someone who's representing your interests as opposed to the builder's.
The last new construction deal I closed this year, the builder had to bring over $3,000 to closing...so my buyer got a great deal. A buyer's agent can help negotiate things for you that the onsite rep may not mention unless you know to ask.
If you don't already have a Realtor, I'd love to help out. Feel free to drop me an email via my website at http://www.wendynormanonline.com.
I'm very thorough and detail-oriented and also have the Certified Real Estate Negotiator designation. Feel free to contact me with any questions. Have a great afternoon!