I've only played since 2nd edition, but there has been a contrast in the way in which supplementary material has been introduced to the game across this time. I remember that for 2nd edition, the core rules were there, with plenty of suggestions for customisation in the DMG and indeed built into the PHB (clerics, I'm looking at you). Supplements that we bought as a group either introduced new classes (Tome of Magic) or modified the fundamental rules of the game to make it more complex (Skills and Powers, Combat and Tactics). There were a few elements of 'splat' to be found in additional spells and kits - we never invested in the Complete X books because they were far more along these lines.
3rd edition came around and the core books also felt complete. However, there was a greater focus on splat, with prestige classes and feats providing an avenue to create and sell new stuff very quickly. The introduction of new elements, such as classes, felt less meaningful because they were chapter 1 in a book full of splat. I don't think they expanded on the rules, or changed the complexity of the game to any significant degree either. Unearthed Arcana was so popular because it finally did this. I suppose you might point at 3rd party supplements, for there were many, as providing this support.
4th edition was the first edition where to me, it felt like they had tried to fit so much into the PHB, they had deliberately set aside portions of classes for later splat books. Like the current trend in the videogame industry for downloadable content, they had already created the additional material, but withheld it as part of a marketing decision. This, as with day one DLC, irks me, because I would have been much happier with four core classes completely fleshed out and books to introduce new classes later. The splat content was expanded a little bit here too, the loss of prestige classes more than made up for by powers, paragon paths and epic destinies. Magic items too became splat, since the crafting system was modified and de-integrated from the magic system. When new classes were published, they didn't particularly innovate mechanically, and it took until Essentials to see that sort of material.
So what I'd like to see for 5th edition is a core that has the base classes fully fleshed out. I don't want to have to buy a splat book later in order to have interesting archery options, or illusion spells. I'd like new classes to use the core systems (fighting and magic, when they perfect them) and throw their own mechanical innovations in where sensible. I'm also looking forward more to books that completely change the game, the famed modules, as I think they offer the most interesting material. Never again do I want a pure splat book, that just gives you more backgrounds, feats and abilities for an existing set of classes - at least offer new mechanical innovation within those classes. If you write a wizarding book, damn well offer entirely new casting systems. More adventures, yes, I want complete campaigns that cover 10 levels, that aren't just railroads - is it too much to ask for something replayable? You can use the same principle plot, but subplots can exist in numbers, and random events might alter the approach of the party to the main plot, or indeed the backgrounds and ethos of said characters might do the same.
