Let's pretend that you've been given the task of creating a demo for a game - what would you do?
Luckman is correct.
For an rts, full game options with a map or two that showcase the engine, not some dinky shit. No twenty minute timers, or training level campaign missions. A real skirmish map that lets people dick around and actually see what the game is like. I can handle not seeing all the sides when a game has 12 or something, but hiding features is only intelligent when there's something wrong with them. When a three faction game demo comes out and only one side is in it, my first thought is that something is severely wrong with the balance, my second is that all three sides play exactly the same.
No screwing with settings either, the demos I've hated the most were ones where some ass stripped down the gui so I had to edit a config file to change the resolution.
For a tbs, I'm a little more forgiving depending on the style, one game of civ would basically be the entire game. If they're based around a campaign, they need to follow the rts rules, if they're based around a sandbox, push it as far as you can. The more I see, the more likely I am to give a shit after I've played the demo. The less I see, the more likely I am to wonder if that's all there is.
For rpg's it all depends on the format, the start of the campaign is never a good idea though. I hate tutorials, unless you break the mold and skip that annoying shit, making it the demo means it has a lifespan of about thirty seconds after I finish playing through the tutorial. Starting somewhere in the middle is almost always a better idea, I've even been suckered into an rpg I hated, and I don't like most rpg's to start with, but the demo was entertaining enough that I gave it a shot. Once I bought the game, the start was so fucking boring I never got to an entertaining part... Give an obvious, non spoiler portion filled with what makes your rpg relevant, usually gameplay and atmosphere are at the top of the list.
Fps, naturally I want to blow shit up. Weapons are a key ingredient, give them. Some people like plots, some people like sneaking around, some people just want to blow shit up. You've designed your game with an emphasis on these for a given market, advertise it. Deus Ex needed a demo that exemplified the upgrade system, Half-life 2 needed a demo that showcased their in game interaction with objects and people, Doom 3 just needed to shove freaky shit at you through the walls so you could rip it to pieces with your boomstick. No gay tutorial levels, no first level intro with dick for weapons access and the lamest badguys in the game.
If it's multiplayer ready, give us a fucking map and server access. Not that I can play them with my 2k ping, but it was a requirement when I was.
I have no opinions on the other genres, I don't like sims, puzzle games, whatever.
Above all else, all of them should be honest. If the Hitman series had come with a fast paced action packed demo, I'd have bought it. It would also be the last Eidos game I'd ever buy. I don't like being a sneaky bastard, I like suicidally rushing into the enemy and being the only thing alive at the end of a level. Or dying, which was often the case in multiplayer. Showcasing the strength of your game means showing them what they can do with it, not taking ten minutes out of a four hour game that happen to be particularly active. When you lie to them, all you're doing is grabbing the wrong audience and pissing them off. As far as I know, selling one game your entire life isn't the goal of the typical developer. The demo should reflect that.