Mixed bag of thoughts below - some of these will not be relevant for an initial visit but more if and when you decide you want to buy a specific property there:
-If there is a detatched garage and/or parking spaces get all the exact details such as freehold/leasehold, what exact land you own, what restrictions are there on nearby spaces etc (in our section of the estate for example there are five garages but only 2 can be parked in front of)
-Any restrictions on satellite dish positioning (if you want one)
-Broadband availability
-Phone sockets and in particular where the master socket is (relative to where you want to position your router) and making sure they don't 'daisy chain' sockets
-Pricelist for 'finishing touches', which will typically run to thousands of pounds (flooring, tiling, lighting, turfing, taps etc etc). You will need to work out what you want the developer to do and what you will do yourself after completion (if you don't need to move in straight away you can probably save on some stuff but arranging it independently).
-Do they do part exchange and if so what is the process (if you have a property to sell)
-Weigh up how realistic a show home is relative to what you are buying e.g. if buying a mid-terrace it may have fewer windows (for obvious reasons)
-What is the position with utilities e.g. does it use an IGT (will carry extra charges from some suppliers)
-What is the snagging process
-How long will the site manager be onsite for after you move in (reason being it can make snagging easier)
-What other development is planned in the area e.g. is that nice field out the back going to be there forever, will a new estate spring up closer to local schools and push me out of catchment area etc
-When will the show-homes go on the market
-Are there any additional maintenance charges e.g. for upkeep of green areas etc
-Council tax banding (tends to be quite bad on new builds.... my house is Band E, which seems quite ridiculous for a 3-bed terrace)
General advice:
-Don't buy all their BS about how they can't give you brochures because they would run out in five minutes, they can't make appointments because they would be turning people away, about how they have dozens of buyers literally queueing up round the block chequebooks in hand the second a new plot is released. The sales patter tends to be quite haughty and designed to make you feel you can't request a significant discount. To be honest at this time of year I would guess they have targets to hit plus typically less people looking to move so don't be scared to leave a sensible offer on the table even if it is lower than asking price. That said, some developments will genuinely be in demand so if you are really keen then make a sensible bid.
-If it is a mature development where previous phases have completed, look online at what properties of the same type have sold for to get a feel for how much discount may be available
-Once you've driven down the price get them to throw in extras to seal the deal i.e. an allowance towards the finishing touches (explain that you can't afford to spend another £5k+ on making the property liveable)