One of the IELTS writing examiners that works here answered this one.
The criteria (not the examiners) is looking for a number of things but here are probably the most important ones.
[slideshare id=74599425&doc=ieltswriting-170407044333]
Task Achievement
- Support for your ideas. If you just list ideas in your body paragraphs, you aren’t developing your ideas. Just add one idea to each body paragraph and support it with an example.
- Are your ideas logical? So many candidates present an idea then support it by talking about something that isn’t connected.
- You need to stay on topic. Something I see regularly is off-topic answers.
Coherence and Cohesion
- Use linking words and discourse markers to make your writing flow naturally.
- Use pronouns, substitution, referencing so you don’t have to keep repeating the same words.
Lexical Resource
- You need to show you can use a range of vocabulary to develop the topic and you need to use it accurately. Lots of candidates think that they can substitute words in English for others from a thesaurus. English doesn’t work like that. Low frequency vocabulary is very highly collocated and this is where lots of students make mistakes.
- Forget the idioms you think you need to use; you don’t need them and you will use them incorrectly anyway (an error)
Grammar
- Can you use more complex structures when you need to? Conditionals, the passive voice, subordinate clauses etc are all examples of naturally occurring complex structures and you need to show you can use them accurately.
- Accuracy is a huge issue for IELTS candidates. So many people think that they get low marks because they didn’t use complex grammar. This is nonsense. Most of the time it’s because most people can’t use simple grammar accurately. For example, articles (a/an/the) in English are different from articles in other languages or maybe your language doesn’t use them. This means that, more than likely, you will be using them incorrectly. Now, the difference between Band 6 and 7 in Grammar is nearly always accuracy. If you have a problem with articles (Maybe you don’t even know that you do!), most of your sentences will be incorrect. This means you can’t go higher than Band 6 for Grammar.