well... aaa there are a lot of factors that get included
is the visibility ==> e.g you can't see a 15.6 " website in only 4" that iphone or some other phone's screen provides you
next thing to include is the protocols ==> a lot of phones get the internet conectivity through 3G 4G connections and all use http or socket protocols which do not differ from computer use but differ in the amount of information they transport. 4 e.g you wanna see a game site which has a heavy flash loyout, it would take a lot of time to load.
3rd thing to mention is the browser... while many of the newest mobile web browsers now support a lot of features, there are a lot of them <specially those created for 3G connections> that don't support the newest features. 4 e.g you try to log in to twitter or idk, when you type your email and pass and hit enter, the basic website sends you to a page with the twitter logo and than redirects you to your profile.
Many of browsers can't support this features like iframe or meta redirecting tag.
there are a lot of other things but this are the most essentials.