pawsplay
Hero
I think a game system is more akin to an Operating System. You don't expect an OS to have everything out of the box and to have no new applications to come out. You want to get new games or new programs for it and use them. If someone told you there wouldn't be coming out much, you would actually say that this is probably not a good OS to use.
If the OS comes with limited functionality and requires a lot of specialized software design to create new functions for, that is also not a good OS to use. There are of course various forms of incompleteness.
Backwards compatibility: This is inevitible at some point, but it's very weird in a D&D game when you can't rework concepts which in theory should be independent of the system. If you were playing a gnome bard, guess what? But setting aside specific races and classes and ability sets... what about all those miniatures? What is shocking to me is that for the time, ever, only a smallish portion of the D&D miniatures can be used in previous games, because so many monsters have changed so much. And the ones that are still there frequently have a new aesthetic. It's not much of an exaggeration to say that you would do as well with Mage Knight figures for a 3e game, and something like Reaper is clearly superior. What is a 3e player supposed do do with a flame archon? Were you hoping for a complete run of 3e demon and devil minuatures? ... Too bad for you. This represents not only a stack of obsolesced books, but of discounted time, since whatever you knew about Forgotten Realms or Eberron or Greyhawk simply cannot apply in the new D&D, with its dragonborn and tieflings and eladrin. High elves can teleport? Who knew?
Incompleteness of design: Every system has things it covers and things it doesn't. With 4e, we've seen a new surge in things in covers, or things in covers in such a way other things become more problematic. I'm speaking mainly about numerous combat options. And things like polymorph or large PCs? Well, the design team decided they were too hard to balance, so they're gone. Virtually any effect that could have unexpected consequences is gone. Look at the succubus charm... they might as well have called it Bigby's Interposing Dwarf.
Incompleteness of modularity: To create a new class for 4e literally requires you to map out 30 levels of stuff, including ideally at least two "builds." A race now needs more than a quick profile... to be balanced, they need PC treatment, especially racially specific paths. In 3e, I could tweak a class if I wanted, but it's hard to imagine stripping down a 4e class and giving it something a little different. Many, many powers presuppose certain weapons or skills or whatever. I've written many variant classes... many, such as turning a barbarian into a corsair, are nonsensical in 4e. To use the OS analogy, the new OS here doesn't come loaded with a lot of program modules and each program has to be written and compiled with a lot of code. New applications are bloated, and tedious to design. Try imagining the class variants section of Unearthed Arcane rewritten for 4e... it would be a 64 page pamphlet by itself. Or, to put it another way, a sizable chunk of Martial Power. Or Martial Power 2.