Really, this would depend on how many dice. Certainly, a larger number of die rolls tends to slow down combat and makes PbP less appealing. Also, more dice generally equates to less casual gamers, in my (limited) experience.
Yes.
A point-buy system makes for better balanced characters, but it does lose some of the excitement of creating your character. However, many people simply reroll until they get a good set anyway, so it doesn't make a huge difference. Optional point buys are useful when people roll appalingly badly.
I prefer classless/archetype systems over rigid classes.
No, especially if characters were flexible enough to become proficient in nearly anything anyway.
Seperate rulebook and worldbook, but a preexisting campaign world encourages players, as it means their GM doesn't have to do too much work to play a few games with the system. After all, you don't want to design a campaign world then find out you don't like the feel of the game, do you?
I dislike reading digital formats, but I don't dislike it enough not to do it. However, I have a laptop and some people don't. Even if only digital formats were released, it'd be nice to print a special hardcover collector's edition at some point for traditionalists.
I hate DRM. But then again, I'm not the one losing profits to copyright fraud. The whole subject is slightly irrelevant, as it will eventually be broken and your rules will become torrents. But surely that's a sign of popularity?