One important point is that you review it from the end users' point of view. It is a must.
Suppose you are reviewing a software, it does not help users if you go on and on about the sort of engine it uses or where the code has been derived from or whether it is open source or not. Yes, you have to mention those points but they won't really help people who actually are going to use that software. Instead, you should focus on what the interface is like, what are the features, whether it is free or paid, comparison with competitors, etc.
Also, before you publish a set of drawbacks for any thing, make sure that you search Google well and those drawbacks actually exist. It often happens that people state in their review that some feature from a software is missing and it later turns out that the feature is right in front of your eyes. This hurts your credibility.
Hope you write some great reviews!