I don't think Windows games have to be different to Android ones - there are certain casual puzzle games very popular on both platforms. Also traditional PC RPGs like Baldur's Gate have been released for Android. (As for Windows 8, the complaints such as the start menu being different or replacing the button with a hotspot aren't to do with laptop versus mobile device - some people don't like the changes, other people do, but the idea of it being unusable on a laptop is a myth, it's just some people like it and some don't, as with any change - there are plenty of other threads to discuss Windows 8, anyway.)

One does have to be careful though - the puzzle games I'm thinking of for Windows tend to be web-based (including Facebook-based). Steam may well be a different market. But then this is arguably all the more reason to support both.

Well, thats only half of the truth...

Even if we overlook the differences in controls (mobile games need crappy touch controls, web games might not have access to more than keyboard and mouse, windows games can use whatever the player has connected to the PC) and graphics / screens (Aspect ratios, GPU Power, DX vs GL, and so on), there are many inherent differences with gameplay expectations on different platforms.

webgames are closer to mobile games when it comes to how people play them and who plays them... its mostly casual games, being played for rather short sessions. Like 5 minute sessions while waiting for the train, or when being bored at work...

That is not true for traditional windows games though... while there might still be a market for casual games, it is smaller than on mobile or web, and most people sit down in front of their PC's for sessions of 1 hour or longer.

a traditional windows game like baldurs gate CAN work on mobile... but it will target more the gamers that started to use their tablets and phones as "game consoles" at home (ugh, but if they like tiny fingerprint-ridden touchscreens, who am I to judge them...). Playing a 50+ hour RPG in short sessions on the move will ONLY work if the game at least gets an updated save mechanic so you can save everywhere, and basically quickstart the game in seconds instead of having to klick through lengthy menus and load screens.

So the audience will be more limited than the one of a game like angry bird.

a mobile game CAN work on windows... its the modern equivalent to minesweeper or the windows cardgames that got famous for being played at work. But again, that is limited audience that is not really using their PC as a fully blown gaming machine, but just wants to play a quick game while being bored.

Now, while most fully blown windows games are just not geared towards short play sessions, most mobile games are not that well suited to being played for 3 hours straigth... yes, I did play angry birds for hours straigth before, but that was while being REALLY bored during my military service. I guess some extreme fan might do it even at home, but mostly these games where built to give you a fun expierience in 5 minutes, which might get very repetitive if played for hours without a break.

There are exceptions of course either way, but they just prove my point:

If you really want to target the core audience of each platform, you will need to tweak even the gameplay of your game a little bit, as the audience has a very different way of playing their games between platforms.

If you want to reach both with your game with minimal changes, your game needs a very special design and be built fur such a cross platform use from the start.