When I wrote my funny situation description last week, I wasn't trying to use 'difficult' grammar or vocabulary. Maybe you read it and thought that it was too 'easy' to get a band 9. If you thought that, you would be wrong!
Here's why my description would get a band 9 (according to the IELTS criteria):
1. Fluency and coherence
The fact that I told a story made my answer really coherent (clear and logically organised). In terms of fluency, my description was written rather than spoken, so of course there are no pauses or hesitations.
2. Lexical resource (vocabulary)
The description might seem simple, but it isn't. A wide resource of vocabulary is used naturally and accurately. There are definitely some 'less common' and idiomatic phrases (e.g. engrossed, the time passed, I suddenly realised, get going, my leg was dead, tipped over, to my surprise).
3. Grammatical range and accuracy
Unless you can find any mistakes, it would be difficult to give my description anything less than 9 for accuracy. The range of structures used is also easily good enough to impress the examiner.
Hopefully this wouldn't be a problem as I'm a native speaker!