Ok, so second of uni is over and you realise what a mistake you made in choosing your accommodation for it. You'll know better next time but you have already spent a whole year living in a sh*t hole, maybe it would have been good to have done some research before hand.
#1 Mould
Most obvious of all but often overlooked, these micro-organisms can be deadly but at least should you be suffering physically from their effects your landlords is required by law (at least in the UK) to rehouse you. Typical things to look for are not walls absolutely covered by it (al though if you see this run!), but small patches in corners, especially in area's like bed rooms, don't feel like you are being rude by taking pictures, it is necessary as often memory does not serve for these small details.
#2 Sunlight
It is best to visit the house during the day to determine the amount of sunlight the rooms are receiving, this is important because this natural light source can have a direct effect on mould growth due to it's ability to heat the house and thereby help remove dampness but also because it is a completely free way to get some extra heat which is always good when on a student budget. The amount of sunlight is often determined by your proximity to other buildings, their height and angle of the house.
#3 Artificial light
If you were like me and lived on a busy road at the front of the house you may have gotten to experience the joy of having a constant bright orange light penetrating your window throughout the night. Eventually this drove me to have to tie a second set of curtains over the first set just to block it and get a good nights sleep. While you probably won't be able to visit the property at night you can usually determine if this is a problem by looking at the proximity of the street lights outside and just by simply asking the tenants if they have any problems with it.
#4 Other houses
Many people (including myself) do not checkout many houses, eager to secure one as early as possible. However this is a big mistake, many people often end up finding out later in the year after moving in that they are paying as much rent as someone else to live in a smaller house, with a worse location. If they had simply looked at more houses this could have been avoided.
#5 Neighbours
It can usually be taken as given the more central you live the more you will have to deal with, however usually this noise is concentrated to friday / saturday nights, not on my street. Our local council had housed many drug addicts / alcoholics on one corner of my road, they would often by quite beggars by day but when night came it was a whole other story, shouting for hours, often the police having to get involved, keeping you up when you have lectures at 9am till 5pm the next day.
One thing you might have expected to be on this list but I would disagree with is proximity to your lecture theatre / university. I lived very close to mine but being so close allowed me to delay leaving the house in the morning, often being one of the last people to show up but somehow living the closest, I have now moved to a house further away but know how long it takes to get there and will just have to wake up a bit earlier.