Having fiddled around with the Starter Set--which isn't hard because there isn't much to fiddle with--I've come to some realizations.
This is not a new edition by any stretch. By itself this product could barely be considered introductory since you can only elevate yourself to level two and the character options are virtually nonexistent: there are no class features listed for any of the classes, and I don't even know if all the powers for the available levels are entirely present. Character creation is based on the choices you make throughout the course of the Lone Wolf choose-your-own-adventure-esque procedure until you have an assembly of words and numbers scribbled on a sheet of paper. Don't get me wrong, I think that Starter Set is an excellent testing ground for people that are considering getting into the hobby. It's cheap and a nice chunk of the product will still see use going forward, but "replacing" the game?
As has been said, the rules and class builds are completely backwards compatible. None of the mechanics differed in execution from other D&D content, and you could plow through the adventure in the back of the Dungeon Master's Book with any class that rattles your dice. Frankly, the only thing that it's replacing is some of the oh-so-obviously slightly reworded monster clones, which is completely fucking fine by me. Out with the goblin hexer, in with the goblin hex hurler I say. Whatever is more fun. I'm sure many DMs have said fuck-all to plenty of "official" monsters for self-made iterations that they find so much more enjoyable. This is not to say that you spent your hard-earned dollars during the initial 4E launch in vain. If nothing else, it provided a stable foundation from which to build your own particular brand of fun upon.
I think to prove my point I'm going to encourage a player to try out the knight or slayer for a romp through Athas along side a fighter, wizard, or whatever. Hell, I might do something really rash and run Marauders of the Dune Sea with only classes out of Heroes of the Fallen Lands.