NB: At this point, I am not talking about works in progress or ashcans (RPGs that are outlined to check feasibility) but those professing to be complete. Scenarios, settings, resources, tools and so on aren't really comparable here because they rarely boast 'completeness'.
I'm going to split up RPG facets into groups: Need, Want, Nice to have. Need items are things that you really can't do without in an RPG. Any RPG, for that matter. Want are those things that I really want from a free RPG, aspects that make an RPG really worthwhile. Nice to have are just that. In the style of 'notes for authors', here are my Needs, Wants and Likes to have.
To be able to print itSo many free games in the past are simple HTML pages that do not print well. Google Documents can take your game and create a PDF from it. It makes all the difference. If you can make an HTML print well then go for it but dual columns tend not to work over many pags.
To be complete enough to playA snappy title representing an important requirement. If a game is advertised as complete, then make it be that. A GM must be able to sit down and run a game from what is offered.
Good spelling and grammarYour readers are going to be suspicious of your game because philanthropy is rare, bump up the quality by good grammar and spelling. Typos can be avoided by spell checking and grammar by reading through carefully.
PlaytestingVery few systems can stand up to purely design. Playtesting is not just about getting the mechanics tuned but also to see if the setting makes sense. If you have made a generic system, then you will need a setting before anyone will play it, so make one and play it. And play it again.
NoveltyHow many fantasy games are there out there? Thousands and many sink without a trace. If a free game is offered for other people to read please make it new in some way. As it's free, you can be as creative as you like: Magic Bunnies in Rockets, Slaughter Nuns, The Journey to The Centre of My Nostrils, My Beard Is On Fire! So please do be creative.
Nice to have
Attractive layoutTwo columns is a good start if you don't know what to do. The eye likes to read along short lines, so please consider two columns. Beyond that, any fancy text layout is is nice but do not lose any sleep over it.
Pretty graphicsIf you can find a kind artist to donate images in a fit of philanthropy or you fancy yourself as an arty type, then please splatter your rules with images. Do not worry if you cannot muster any up, the game can stand on its own. If you do have strong images with the game, more are likely to take it up as images can spark the imagination instantly and without effort, whereas text description requires more effort of the reader. Images are never missing from a free game.
I suspect that there are more aspects a free game writer should consider when sharing their idea. Such is the danger of any list, it is never complete. Please do add your thoughts on items missing, or perhaps a different ordering. Do you feel that I've downgraded something important to you as a 'Want', when it should be a Need? Please do let me know.