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Tenses, good vocabulary, opinions, extended sentences, accuracy, variety.
I concur, make sure you use as many tenses as possible (present, perfect, imperfect, future x2, conditional + perfects) and other "complex constructions", even if just one or two examples of each. Opinions and justifications (J'aime x parce que c'est y etc.) are also key, try to get in at least 5. It will sound slightly artificial but will gain you access to the top marks for range.
Also, when you sit the controlled assessment make sure you thoroughly check for errors, any verb error and they start to dock marks. I presume you have a dictionary (no idea what board you're doing - I did WJEC) so make use of it to check spellings (and conjugations if it has verb tables), do not use the dictionary to come up with whole new sentences during the assessment!
Even though you're GCSE, you're still expected to incorporate various tenses into your answers so make sure to use the subjunctive, the imperfect, perfect, conditional and future as well as the present. The subjunctive, you most likely won't be taught because it's mostly A level, but that doesn't mean you can't use it; a simple 'bien qu'il soit' is sufficient. Idioms are great too, simple things that you can apply to a wide range of topics. Sentence starters, good concluding vocabulary and connectives are key... simple things like 'On exagererait a peine en disant que...' (apologies for the lack of accents, laptop isn't playing game) which means 'It's hardly an exaggeration to say' - not only does this sentence use various tenses, it's also very advanced and will impress at GCSE! Personal involvement (although don't get TOO carried away with this.) In any case, if your grammar isn't good, the less you write, the lesser the chance of mistakes. If it is good, write till your heart is content. But try not to waffle Certains pensent que...mais, en réalité... - examining both sides is good too. I know that's a lot of stuff, but don't panic. Having been through it myself, it pays off in the end. Even if you have difficulty with it, there's no harm in learning a few sentence starters that have the more complex tenses in them even if you don't know exactly what they mean in English - it's not like the oral where you have to bounce off the examiner. Although your knowledge should be varied so that you can adjust it to what you need. CCEA has a booklet of GCSE vobulary that's very handy to have. Best of luck!