Less a manifesto, and more a notverymanlyfesto, as this is very much a tech-centric list. If you want thoughtful game theory, you've got the wrong nitpicker.
The PC is the best gaming platform in the world - but it could be better still. While it's great that the PC doesn't have to suffer quite the same degree of standardisation as its locked-down console brethren, we have nevertheless fallen into certain patterns of how we game. There are things we take for granted and thus expect, like WASD controls in FPSes and patches for bad bugs. There are others still we should be able to take for granted, but can't because the same damn-fool oversights happen again and again. Even outside of the more obvious annoyances like referring to Xbox controls or including ridiculously draconian DRM (which are both more a question of money than of thoughtlessness), a ton of stuff that any gamer could have told the developer was a glaring screw-up keeps on turning up in otherwise great games. Here are just 10 of the worst offenders, 10 things that every single modern PC game should get right and has no excuse not to. Please do suggest others in comments below.
1. Alt-tab support.
Perhaps the single greatest, but so often neglected, Must-have there is. Just having rudimentary task-switching support in there isn't enough (hello-o Valve games) - it needs to be fairly quickly and smooth, and included in the original release of the game, not in a patch down the line. This should be as big a priority as graphics or sound. Don't care if it's a massive pain to code in. Don't care if you have to re-start the entire game from scratch to put it in. Alt-tab is absolutely integral to the way we all use our PCs. Half of us essentially live at our computers - we need to be able to task-switch to an IM window or an inbox or even another game in moments, not be locked into one program. Frankly - if your game doesn't alt-tab, it's not really a PC game.
Possibly deserving an entry of its own, but in the name of keeping this list to 10 I'll include it here - all PC games should be able to play in a window. I've missed social events because someone's instant messaged me about going to the pub, but not bothered to phone or text when I don't get back to them right away because I'm off in a game. One day, the girl of my dreams will magically message me, and by the time I've exited the game she'll have got bored of waiting and declared her love for my arch-nemesis (I don't actually have an arch-nemesis, but I'm working on it). Then I will hunt down and kill the developer of whichever unwindowable game I was playing at the time. They will appreciate why. Window play is also necessary for 2D games whose resolutions can't be changed - 800x600 pixels of pretty hand-drawn art look like roadkill in toontown when they're stretched over a 1680x1050 panel.
2. Use standardised install and savegame folders
Everything goes in Program Files by default, please (and, just as importantly, there needs to be an option to install anywhere the player would rather). Don't have your game install itself into the root of C:\ or an obscure sub-folder, and when you do put it in Program Files don't stick it inside [Publisher name]\[Developer name] - just stick a folder directly in there under the game's name. Gamers want to be able to find their game files easily, not have to Google for everyone involved in its creation just so they can work out what folder it's in.
This is doubly true of savegames. We need to be able to back those suckers up in case of disaster or a Windows reinstall. Know where STALKER hides its savegames in Vista? C:\Users\all users\documents\stalker-shoc, that's where. Here's where games whose developers aren't crazy stick their saves on my PC - C:\Users\Alec\Documents\My Games. In other words, the standard My Games folder inside (My) Documents, a two-click, standard process to reach. To find STALKER's saves, I have to dig through five separate sub-folders, in something I'd never otherwise look at. Who are these mythical 'All Users'? They're not me, that's who.
Even our beloved World of Goo fails at this. The game goes into Program Files\World of Goo. The savegame - and the savegame alone - goes into C:\ProgramData\2DBoy\WorldOfGoo. ProgramData? Worse, that's actually a hidden folder by default. Gah!
3. Automatically set themselves to your desktop screen resolution
Don't default to something horrid and archaic like 640x480. The vast majority of PC gamers use flatpanel monitors, and games running at anything other than their native resolution tend to look horrible. Save us the hassle of changing the setting ourselves, but most of all save the less tech-savvy from having to work out what a resolution even is in the first place, or just putting up with a blurry screen because they've no idea how to fix it. Clearly, still allow the resolution to be easily changed to whatever the gamer wants, however: the game needs to support every res the monitor does.
4. Support widescreen resolutions.
Widescreen isn't the future - it's the present. Just look at the consoles for proof of that, or at the top hits for 'monitor' on Amazon. And expecting us to edit an ini file or type in command lines doesn't count as widescreen support.
5. Uninstall in seconds.
Don't have it laboriously check every single damn file before it has the grace to remove 'em - just wipe the folder, pull the main hooks out of the registry and be done with it. I uninstalled the FIFA 09 demo today, and it all but locked up my PC for ten minutes while it did its ridiculous, disc-churning thing. Then I uninstalled the King's Bounty: The Legend demo, and it was gone in the blink of an eye. That's the way to do it. When I want someone to leave my house, I just want them gone - I don't want them hanging around on the doorstep making tedious chit-chat for half an hour. Tied into this is installing neatly in the first place to ensure removal is simple - the game should all end up in one place, not explode tiny bits of itself all over the hard drive.
6. Don't require the CD/DVD in the drive to play.
Again, we're talking about a PC, a device with hundreds of gigabytes of storage. A game needing to look at a plastic disc entirely external to the game install folder whenever it runs is openly ludicrous. I know it's for copy protection's sake (and even so is of debatable effectiveness in this day and age), but the annoyance to legit customers surely outweighs a few extra lost sales before the inevitable no CD crack turns up anyway. Requiring PC gamers to scrabble through a vast pile of discs just to play the game they've already installed is contrary to the nature of the platform, and lures people towards less than legal solutions that may ultimately push them further towards piracy. And you wouldn't want that, would you publishers?
7. Keep the quicksave and quickload keys far apart.
Accidents happen, whether it's sausage-fingered gamer stereotypes or just furious keyboard-slapping in rage at another defeat. Hitting quicksave when you're reaching for quickload is the worst thing in the world, including being licked to death by a pack of hobos. If you set quicksave and quickload to F5 and F6, you are not fit to be developing PC games. F6 and F9 are fine - that's enough space to blame quicksaving just as you get killed on the player being stupid, not on developer thoughtlessness.
8. Escape means menu/pause
The button's actually called 'Escape', for heaven's sake. Why on Earth would a game ever bind a request to leave or pause the action to anything else? This needs to be standardised. No-one wants to be miserably jabbing at random buttons one-by-one because the phone's ringing but they've got no idea what brings up the pause menu.
And, because I want to keep this list PC-centric rather than generalist to all games, I'll mention cutscenes here rather than as a separate point. Pressing Escape during a cinematic means I want to end that cinematic. Literally, I want to escape this movie you are making me watch. Please respect that button's purpose. Please respect your players - and if you make any of your cutscenes unskippable, you don't.
9. Auto-backup quicksaves
Again, accidents happen. Excited gamers hit quicksave when they think they're out of danger but a giganto-beast is just about to feast on their ankles. Files get corrupted. And then you're screwed, with no option than to rewind potentially hours of progress. So whenever the player hits quicksave, the game should keep a copy of the last one in case of disaster. The last two, ideally. It's just common sense, and surely an incredibly simple process.
10. Patches should fix, not break
If your patch renders savegames from previous versions of the game inoperable, it's just not ready for release. If people have to restart a game from the very beginning because of this, they will hate and distrust you for it. If there's honestly no way around this, because the under-the-hood changes really are that absolute, then the patch needs to say as much in giant red letters when it's run: "INSTALLING THIS WILL BREAK YOUR SAVES. OK?" A footnote in the readme file is not enough. Better yet, the lead designer should show up at the door of anyone installing the patch with a box of chocolates and an apologetic hug.
Stepping away from savegames, if your patch introduces new problems then it's hardly a patch, is it? Test it to death before you let it into the wild - remember that Eve update which deleted critical Windows files? Such a thing cannot be allowed to ever happen again.