I prefer a slimmed down ruleset rather than "core only". The 4 main classes: Fighter, Wizard, Cleric, and Rogue all have very general roles, are highly customizable, and therefore can be chosen by anyone. All other classes are "Optional" and within the DM's purview.
To use an optional class you and the DM either build it or agree on a published one. These classes are then customized into your world based on its own internal logic. The paladin class would belong to a specific order, assassins may have been taught within their own guild, bards are taught in a specific tradition or school, etc. A separate bardic school (from a different culture perhaps?) may develop something called "bard" as well in game, but it would be somewhat dissimilar from the first and use an alternate class design. In other words, paladins can be both a prestige class and a standard class. As long as all aspects of the rules adhere to some valid in-game logic, I see no problem with this.
EDIT: I find pretty much anything ever published can be used with this method. So I guess I am rules light & option heavy.
In my view, customization is the name of the game. Creating your own stuff is the Core Rule. Designing custom feats, spells, monsters, magic items, et. al. is central to playing and enjoying the game. The laments of "core rules only" being boring assumes nothing is changed. It's as if everything you encounter in game will be the same as what you have read earlier in the books. What fun is that? It hardly qualifies as exploration or "fantasy" in my mind. Nothing is then fresh. And I think it will lead players into acting on assumption and metagame knowledge rather than curiosity and trial-and-error.
