High Resolution (i.e. 800px+ in width, often full-browser or even full-screen)
- More immersive.
- Occupy a decent percentage of a high-resolution monitor.
- More room for gameplay elements and user interface.
- Greater detail possible in artwork.
- Artwork right-size to port to console
- Slower performance.
- No keyboard support in full-screen before FP10.
- No room for anything else on the page.
- May not actually fit on monitors of netbooks / older computers.
- Much faster performance / less variation between new and old computers.
- Leaves room to sell advertising around the game (decide for yourself if this is a good thing).
- More flexibility for use on HTML pages - e.g. "Add this to my site" embedding etc.
- Pixel art looks better in low-res games.
- Artwork right-size to port to mobile / iPhone
- Games get completely lost on high-resolution monitors, especially laptops where the pixel-density can be much higher (e.g. on 15inch screen with a 1800 pixel width, the average Flash game is only a couple of inches wide)
- Less immersive, further away from the console experience
But hold on a minute! Flash is a re-scalable technology, right? So can't we make games that work at any size? Well, yes and no. Vector graphics scale nicely, but performance suffers at higher resolution. Bitmaps scale up smoothly but aren't so great scaled down (Remember to turn allow smoothing on in your library and/or code!). For pixel art there is the option of turning smoothing off, too. There is hardware scaling on full-screen mode, but text and some graphics start to look a bit strange to me. I think the ideal situation might be to be able to build your game at 1280x720 (720p) and scale down to the correct window size. This is how XNA handles both HD and SD TVs when publishing for Xbox. I don't think the current generation of computers is quite up to this in Flash though, without hardware acceleration. In general Flash does do a much better job at upscaling than you would expect, and if you build with rescalability in mind, it's a bit more work, but you can generally pull it off. Portals should maybe start offering the option to have alternative resolutions - like Flashkit used to!
Do we mind playing tiny games on our huge monitors? Should a Flash game be thought of more like playing a gameboy or iPhone within a window than a playing a traditional PC or console game? Leave your thoughts in the comments please!