I view this in the opposite light (probably because I play Savage Worlds - a game reknown for exploding dice and cards). If you are going to make a generic format, then you have to consider these items up front. If you are going to do them last, then I may as well use Google+ hangout (I think there is a app floating around to do a basic battlemat these days).
As part of the snippet you quoted, note that I also said Exploding Dice could be modeled with Macros. Making it a junior dev or end user task, not a senior dev task.
Or, pick 3 or 4 popular games and have overlays that does the basics of those games well. Or pick one game system and nail it so even a n00b can just log in and play and let people clamor for you to do their system of choice.
I suspect Roll20's approach is to not touch specific game rules or unusual mechanics and be agnostic by not touching any game rules. You don't NEED to have your character loaded up into VTT to play a game. And technically, more RPGs have simple dice mechanics like D&D than not. So effectively, they HAVE chosen 3 or 4 popular games. All the ones with mechanics that can be expressed with basic dice notation as (XdY +/- V)
It may be considered that the very act of having game mechanic support is what is driving up the complexity of VTTs. Get it back down to what my kitchen table does, hold a battlemat, and present all the players together, and make it easy to communicate to each other.