1) You're vulnerable to the upper hand of an agent who's representing the seller. If you start talking about how much you love the place and your need to move in the next 30 days, the seller's agent will definitely "have your number." These are not inherently bad or unethical people, but they are professionals who know their mission - to sell the seller's or developer's units for the highest price possible.

2) You may make it impossible for another agent (who'd be working on your behalf) to negotiate for you if you decide on making an offer later. Many open houses and sales centers have strict policies about any buyer's agent being disclosed and registered in writing on the buyer's FIRST visit. This is to prevent a scenario where the seller's agent does all the work, only to have another agent come in at the last minute and collect 1/2 the commission. So if you fail to let the seller's agent know that you're represented, you may be locked into working with him or her exclusively on that property since they often will not pay your agent's commission.

3) You may end up paying more for the place you love. This can be particularly true if you're interested in a new development. Pricing, incentives, and upgrades are often negotiable, but the sales center staff isn't likely to point this out to you. Even if you try to negotiate some extras for yourself, you likely won't have the benefit of perspective. Active real estate agents know what's going on in the market. Your agent may know, for example, that a certain developer will offer certain perks if you ask. Without that representation, you'll never know what kind of deal you may have gotten.

4) You could blindly buy into the "newer is better" mentality. New construction sales centers are intoxicating places with great lighting and pieces of expensive stone and imported wood hanging around. They're like new car showrooms as far as the "ooh and ahh factor" is concerned. But shiny appliances and immaculate floors aside, buying new construction is not without its share of risks...like being unable to compete with "Phase II" when you get ready to sell in a couple of years. You'd be wise to consider all this before signing on the dotted line. Get a pro who's on your side to help you think it through.