Although I've basically switched over from classic Dungeons & Dragons versions to Pathfinder, I spent a few moments earlier this summer browsing some old D&D materials, and the classic D&D clone Labyrinth Lord.
One of the best changes made in 3rd edition and later versions of D&D is undoubtedly the simplified attack rolls. All those tables of target numbers and THAC0 calculations and stuff in classic D&D feel so cumbersome now. So I started thinking about how hard it would be to convert the classic D&D combat system to use the simpler d20 System mechanism. And while examining this I realised both systems are actually based on the same underlying principles.
The only difference is in the way data is represented. In 3rd edition the AC is the target number for an attack roll, while in classic D&D it is an index number used to look up the actual target number from a table. Classic D&D also calculates level advancements directly into this target number, while in 3rd edition it is presented as an individual attack bonus.
Converting AC into a target number and calculating a character's attack bonus can actually be done with pretty simple formulae. So I set out to do so, not for any real current need, but just in case I ever wanted to run a game using classic D&D rules. And I also wrote down the formulae and made tables of each class's attack bonuses in an article in my RPG section, should anyone else be interested.
I considered doing the same for stuff like saving throws, but realised it wouldn't have much real benefit aside from superficially unifying roll mechanisms (whereas converting AC to target number does actually make combat easier when you don't need to keep track of different target numbers and players only require an easily memorised attack bonus for their rolls).
Of course I wouldn't be surprised if someone has already done something like this, I was just too lazy to search for it...