Nothing, seeing as there is only one game on Steam I'd be interested in buying and I believe it's original amount is overpriced due to it being in alpha and so far the sale for it has been 10% off
Before people assume I am being ungrateful, I am not claiming they are greedy and the sale "isn't good enough", it's just that £20/$30 for an alpha stage indie title is too much for me to be willing to buy. If they do a 50% off sale I will happily buy it, but to be honest I am not in any particular rush.
There is also the fact that most sales tend to go up to 50%-75% so if it's less than that, it is normally a good idea to wait.