Comments concerning feasibility have mostly been omitted as cycle-specific. There is a widespread belief that writing HST proposals is difficult and time consuming, and may not be worth doing given the heavy oversubscription. The abstract is perhaps the most important part of the proposal. But panellists do demand lucid applications. Correct grammar is not sufficient in itself.
Clarity is essential.
If the applicant manages to crystallize things for me and I see clearly what the goals are, then I am much more likely to think this is great science than if I have to use a can opener to get to the essentials. Keep the text of the proposal short and sweet and to the point. Each reviewer is faced with plowing through an enormous amount of tiresome verbiage and can therefore only devote a minimum of time to each proposal. Proofread carefully for typography and spelling. Sloppy typesetting and spelling conveys disrespect for the selection process, and is insulting to the reviewer.