I like the idea of classes you have to qualify for, with bonuses and abilities that stack (though WFRP did the idea first). However, I think the d20 version wasn't properly implemented until d20 Modern. The breakdown of baisc class (simple core concept), advanced class (specialized concept, available at 2nd or 3rd level) and prestige class (rare specific concept tied to an organization or power, not available until 6th at the earliest) works much better than the D&D version. Especially since d20 modern Advanced classes are all built on much the same platform (bonus feats at certain levels, special abilities at others).
Given the bard/ranger/paladin prestige clases available in UA, it's easy enough to set up a campaign to do this, but I bet we'll see WotC go that route as well for 4e D&D. d20 Modern is obviously a testbed for d20 ideas (look how many canges between 3.0 and d20 Modern were incorporated into 3.5), and the basic/advanced/prestige class breakdown has been used well there.
Given the bard/ranger/paladin prestige clases available in UA, it's easy enough to set up a campaign to do this, but I bet we'll see WotC go that route as well for 4e D&D. d20 Modern is obviously a testbed for d20 ideas (look how many canges between 3.0 and d20 Modern were incorporated into 3.5), and the basic/advanced/prestige class breakdown has been used well there.