I've just read a guide to choosing your tent which I thought was very well thought out and informative, however, one thing which I found was missing from the guide was information about waterproofing.
Some people think that all tents are waterproof, which to an extent they are. But, there are degrees of waterproofing.
Most tents which are brand new or nearly new, should show how much PU coating is on the tent. This tells you to what degree the tent is waterproof. In brief, anything with a PU coating of less than 2000mm will not hold up to anything more than light rain. Nobody wants to camp in the rain....but it happens to the best of us! If you want your tent to hold up in heavy rain, then you should look to buy a tent with a PU coating of 2000mm or more.