4ed tried to make encounters simpler and quicker, giving a kind of MMORPG feel to it. How do your attempts compare to those? Same or different focuses?
I'm afraid I'm not very familiar with 4ed rules, so I can't speak very authoritatively on this. However, I don't find that the rules or the classes feel very MMORPG-like, to the extent that I understand what they are like. One focus here was more on making it easier to create the character you want, without sacrificing utility. I recommend going to the thread and asking these sorts of questions there.
Is this based off a general d20 system, or the 4th ed rules, or the 3.5ed rules?
I'm just curious. 
I wasn't involved in the early stages, so I'm not 100% sure, but I think more or less 3.5ed in spirit, but d20 in practice, due to the legal issues involved (d20 is Open).
After years of not playing, I've started playing Pathfinder D&D (a 3.5 spinoff) with my friends. While I did spend time making sure my characters were useful (my current one is a sorcerer), I never really felt that casters were that OP at higher levels, at least when it comes to combat. Many high-CR monsters have crazy saves and SR, and many high-level spells have no or almost no effect when the monster succeeds on the saves. True, many do threaten with OHKs, which can be a bitch for the DM to handle, but I find that enemies resist my spells quite often despite my specializations and feats to counter SR.
I only started hearing about Pathfinder when I became connected with the Legend folks, so I can't really speak to this, either. However, several of them have brought up specific and general issues with balance in Pathfinder that I can neither recall nor scrounge up right now, that Legend fixes.
Again, I recommend asking on the Reddit thread, where people more knowledgeable than I can respond.
