Everything. See, the GRE essay is nothing more than a setup. You're given a point of view to expound on. Let's take an example from ETS:
“Some people believe that corporations have a responsibility to promote the well-being of the societies and environments in which they operate.Others believe that the only responsibility of corporations, provided they operate within the law, is to make as much money as possible.Write a response in which you discuss which view more closely aligns with your own position and explain your reasoning for the position you take. In developing and supporting your position, you should address both of the views presented.”
You can write a solid GRE essay with any of three approaches: 1) Argue strongly for a socially conscious corporate philosophy; 2) Argue strongly for a revenue-oriented corporate philosophy; or 3) Argue strongly for a middle ground – that corporations are well within their legal and ethical rights to generate as much profit as they can, provided they do so in a socially and/or environmentally conscious way. (take care that if you go with Option 3, that you are very clear in the opening paragraph that the middle ground is your point of view, and that you're not just tossing out ideas)
That's where the editorial approach comes into play. Whatever you're writing about, no matter how mundane the topic, try to work yourself up a bit. Try to become impassioned. Try to think about aspects of each point of view that might make you angry, or excited, or forceful. Does the idea of businesses acting against the interests of their communities infuriate you? Then perhaps you'll go with option 1. On the other hand, perhaps you know people whose businesses have been hampered by government interference and that bothers you—if so, option 2 might be the way to go. Otherwise, maybe you're impassioned about the need for a nuanced, balanced approach to these hot button issues, and you want to convey that. Option 3 is probably the way to go in that case. (take a look at some ideas for expressing opinions here). Moreover, with the GRE increasingly accepted in place of or alongside the LSAT for law school applicants, the importance of solid, analytical opinion writing becomes even clearer. See this article from USNews for advice on choosing which exam to take as a prospective law student.
Remember, though, in all 3 cases, you're still ‘Analyzing an Issue’, and you'll always need to consider counterarguments. But the best editorials are still deeply analytical, even in their forcefulness and passion. This is what you should strive to do in the GRE essay—to get the reader as involved in reading your essay as you were in writing it. But don't take my word for it—pick up a newspaper tomorrow morning, and read theditorial (or op-ed) section, and see for yourself. It's excellent practice for the GRE.