Grammar and spelling are minor problems. If you have a powerful grammar corrector on tap, you won't even have to bother too much with them. The following is a list of the most common problems students usually end up with in their essays. Next time you're preparing one, make sure it doesn't suffer from any of them.
There is no thesis. A cardinal sin in essay-writing, you should make sure you have a main point. Otherwise, all the work you do the rest of the way won't matter.
The thesis is too general or too narrow. Cover too much area and you'll have a hard time supplying sufficient arguments for your thesis; cover too little and you won't fill enough space for discussion. Find a middle ground that coincides with your word count requirements.
There is no sense of direction. If there's no innate reason for one paragraph to follow another, then your essay is suffering from this problem. To fix it, rearrange your ideas so that they develop into the conclusion you intend to make.
There is a lack of adequate transitions. It's not uncommon to jump from one idea to another throughout the body of an essay. That's provided that you supply adequate transitions to handle them. If you don't, there's a good chance the reader won't be able to follow how your writing builds up.
There are too many generalizations without valid accompanying support. Any time you claim something that isn't a fact, make sure you support it with valid reason and evidence.
The introduction or conclusion isn't strong enough. Always put extra work in your introductions and conclusions. They're the first and last things a reader will see, so make sure they leave the right impressions.