1. File uploads on iOS is not supported as per standards. Android is making a difference here.
2. HTML5 Full-Screen API is not supported on mobile browsers. On the iPad, Safari doesn't switch to full-screen mode at all, whereas on the iPhone it does.
3. Available memory is very very limited. About 5mb or so. So you can't load too many images on the same page or it'd crash. Not even a complex webapp to be frank.
4. Number of ports per connection on mobile browser are fewer than on a desktop browser. So a complicated/heavy webapp isn't going to make a cut.
There are other serious limitations too.
Like you can't tell the browser to remain in landscape or portrait via your site if you wanted to. CSS properties like position:fixed or z-index and binding of events on the layer with highest z-index is buggy or simply doesn't work.
Other options like raw in-line base64 images inside CSS are not rendered properly etc. etc. etc. The list could go on ... :)
