Hopefully you know about the area you are buying in already - you should be clear about how you're going to work, where the shops are etc etc before arranging the viewing. Make sure you like the area and will be happy walking home at all hours of the day. Go in with an open mind - if the location is right most things internally can be changed.
One thing I always check is the water pressure of the shower in the bathroom - generally it either has enough pressure or it doesnt and theres not much you can do about it.
Other than that make sure you have an idea of storage space. Are there enough cupboards for your items, or could you install them if not? Would wardrobes fit in all the bedrooms? Is there a loft / other storage area for stuff that you only use occasionally. Is the loft convertible? Or if it has been converted how secure is the flooring etc?
As others have said think about practicalities - does it have enough plug sockets, what age is the boiler - both of these can be easily remedied but will cost. In your mind think about what you need: new plug sockets are around £200ish to install, plus cost of replastering/painting, boilers are £2000+.
And check out the garden - is the grass growing, is it well cared for? If its North facing it can be hard for grass and plants to grow. Is it overlooked - will it be pleasant to sit out in?
Also play it cool with the estate agent - dont reveal your total available funds and dont reveal any deadlines with mortgage offers etc under your offer on the house has been accepted. And dont be overly enthusiastic about the place, always say youve got more to look at. Remain sceptical about any claims they make.
Finally dont go in at the full price - go in 10% less if you really want and make your way up in stages toeards the asking price.