So this seems to be roughly what we have so far:
- The Mearls Legacy (The Book of Iron Might, Iron Heroes, The Book of Nine Swords): The big thing here is the basic idea that warriors classes have often been neglected in favor of spellcasters who always seem to be given a new way to cast spells or spells upon spells to choose. The warrior classes did not have really anything that engaged them into the play or made them feel as epic as spellcasters. This was touched upon in BoIM and then brought to the forefront in Iron Heroes. It also greatly increased the amount of per encounter abilities, created a feat tree system, reducing the reliance upon magic for heroic characters, and the token system which rewarded players for players fulfilling their combat roles (the berserker is supposed to take hits, the hunter is supposed to survey the area, etc.). And much like AU/AE it created its classes based upon various archetypes of warriors and rogues which players created. The magic system was nothing really to write home about, but that was mainly because Mearls did not have adequate time to hammer it out before going to Wizards of the Coast. The Book of Nine Swords integrated some of these ideas into "standard WotC D&D."
- Arcana Unearthed & Evolved: Again, this system tried to redesign classes not based upon sacred class cows, but based upon common roles and archetypes of players. It created a universal spell list (no real distinction between divine, arcane, and psionic) and an incredibly flexible magic system that while deviating from Vancian magic, was still was close enough to the existing D&D system to be recognizable. Furthermore, the lack of the standard races and racial levels proved to popular and were brought into other D&D products and later influenced the development of racial substitution levels for classes. Also many of these classes were notable for being incredibly flexible in their design to the point that it was easy (in fact DMs were encouraged) to create new versions of the Champions, Totem Warriors, and Witches. Arcana Evolved was probably more influential than the Epic Level Handbook in terms of play over 20th level in which AE just raised the cap of classes and gave the classes new exciting abilities that made 20+ play look like a natural extension of the class instead of the clumsiness of the Epic Level Handbook.
- d20 Modern
- Blue Rose & True20
- Star Wars Saga Edition
The above systems can almost be talked about together because they all were greatly influential upon the desire to simplify or expand the mechanics of the system. d20 Modern was the first of these new systems and was important for shaking the system up with generic classes and advanced classes as well as the important talent system. When Blue Rose was created for Romantic Fantasy, it brought about a completely new system that reduced all rolls to a d20, simplified classes to Adept, Expert, and Warrior, removed HP, simplified skills and the feat/talent system of d20 Modern and class abilities, and was just an excellent system of d20 reductionism, which was later cleaned up for more generic play in the True20 system. SWSE built upon many concepts which were familiar in both d20 Modern and True20 and as has been said often in this thread, effectively served as 4E Beta.
- Eberron: Eberron creatively shifted the existing material of D&D in new and exciting ways much in the same manner of Dark Sun.
- Unearthed Arcana: Much like AU/AE, this was WotC alternative means, methods, and mechanics book that allowed for designers to see what mechanics introduced in the book players more readily introduced into their own campaigns.