...doing this is much harder in 4e than previous editions. Consider what that means for the different systems. If I want to adjust magic, or combat, or any other feature of the game....I can take 3e and do that by adding some feats, changing a few rules about initiative or combat casting, etc. Because of the structure, I can make changes to the tone of the game by directly addressing it. Now to do the same in 4e...well just consider what it would take to alter the tone of melee combat....all those powers to review and modify... In which system is it easier to generate a new class? If, as I am often informed, the powers and their functioning (all the X's and O's) is a necessary vehicle for the 4e architecture to convey tone, reworking the tone of melee combat would require examining and rewriting hundreds (thousands, by now?) of melee powers in multiple classes. I think sheer proliferation of classes, feats, spells, etc. in 3PP and on the internet argues for 3e/d20 there. In the older systems, things are even less structured.
Generating and/or modifying 4e powers and systems is actually not hard; I think what you are sensing there is the natural learning curve with a new set of mechanisms. When AD&D came along I felt very intimidated from modifying or inventing new system material for it - especially after reading the dire warnings about "upsetting the delicate balance of play" in the DMG. 4e initially looked at least as intimidating, but after taking time to 'grok' the system I now find it pretty easy.
As for making the "Martial = mundane, magic can do *anything*" game some seem to desire out of 4e, I would say it could be easy or hard, depending on your approach. If you are happy to abandon balance it could be really easy - just jettison all the effects of Martial powers that you imagine to be "unrealistic" and make Martial power users make extra rolls for any effect that could be useful. If you want to maintain a balanced game for all classes, however, it would be hard - but that's not specific to any particular system, it's because "Martial = mundane, magic can do *anything*" is inherently unbalanced and nothing very much can be done about that except possibly making magic much, much harder and more dangerous to do.
As an aside, I finally got to play FATE this weekend - yay! I would say I think the same applies as with the D&D examples above; from limited exposure I cannot see how one would even begin to make a "dungeon crawl" aesthetic using a system that so boldly encourages the players to drive the story. Not that I think it's not possible - it's just that I would need to grok the "working parts" of the system far more than I do now in order to see how it could be achieved.
Yes. Once the basic idea of an rpg was out there, people started making other versions to better address things they wanted to see (with wildly varying degrees of success). That process hardly stopped or even slowed when 4e came out, so I'm not sure what you think it proves. Its not like 4e came out and suddenly all the other companies and indie designers closed up shop crying "Finally we have found the perfect Role-playing architecture!"
Um, I'm not sure where this came from - Abdul's point seemed to me to be that there is no such thing as " the perfect Role-playing architecture", which is why a game that tries to be "all things to all men" won't work. I'm inclined to agree.
As a final note (not specifically to you, Ratskinner) about the vaunted "tactical system", it's been said before but I think it misses the point. The main things I value about 4e I think are the clarity, the elegance, the structure, the precision and the rigour of the base rules. Various - often very different - systems have this; Primetime Adventures, Universalis and DragonQuest are three that spring to mind, but DDN does not. And these are things that, if they are to be present at all, have to be baked into the core of the rules, so it's pretty clear that DDN is unlikely to have them added in by any supplementary or "modular" system later. In this respect I don't think DDN is going the wrong way for "everyone", but it will certainly fail for some, namely those who don't want to be making up the fundamentals of how the game works as they go along. Maybe the advantage of that is that you get what you think you want, but the disadvantage is that what you think you want isn't always good for you...