Class 6 designates write speeds (to a clean card) of at least 6MB/s. Class 2 and 4 are similarly defined. There are no classes higher than 6; so whether it's 6 or 12 MB/s it's still Class 6... It actually say nothing about read speed; read speed is always much faster than write, but need not vary much between low-end (non-classified) and Class 6. In practice, a cheap card is a cheap card, and generally slower reading as well.

Exactly. Ideally, that wouldn't be necessary, so it qualifies as "just a touch slow" IMHO.

Mainly it relates to the on-screen keyboard; a lot of people have trouble with doubling, and also it senses whether to bring up the finger-board or stylus-board depending on pressure, and that's pretty bad right now. Most people have to press/thump/smite the screen rather harder than normal to summon the finger-board. Both issues are largely fixed in Diablo, though, so you don't really need to worry.