I agree with most of your sentiment, and that is part of the problem. Almost nobody plays D&D like this, tabletop or even virtual tabletop/realtime-wise online.
Even with a forum game, you're still "using the full system", though you're leaving out some sizable chunks here and there, for the sake of time and expediency and because it would require GM-Player back-and-forth. I just don't see D&D, or indeed many game systems, including ones I like, lending themselves well at all to pbp at least, because of the sheer size and detail of their system - online forum games are a place where less is more, you want a small, simple system that requires as little input from the players as possible, and as little recordkeeping from both GM and players, in my opinion.
Games with characters with a few dozens skills, that's a lot, add in special abilities or feats, a stack of weapons with its own stats, then rules for movement, encumbrance, and THEN combat rules and its specific subset of movement, special attacks and maneuvers, etc. I don't see how, and I don't see WHY people even TRY to use a normal existing system for forum games - it is like fitting a DVD and high def TV in an Edsel - yah you CAN do it, I guess, if you want to - but WHY? They obviously aren't made for each other.
That's why I like Wushu, which to me is almost MORE suited to forum games THAN tabletop, though it has plenty of players that love playing it face-to-face, but it lends itself very well to quick online play, being a tiny system with a narrative-based mechanic for dice.
Past these - I think of more serious investigative games like Call of Cthulhu, that has like 100 skills, with which the characters have usually at least a 01 to 05% chance of success, plus the ones they put points into, plus stats, plus weapons, etc. It is a fairly tight system but there is just so much OF it; I played in a forum game of it also, and was equally shocked the guy claimed he was using the whole system - I just don't think it is sensible to do that. The same would be true of Warhammer or the D6 system or Tri-Stat or anything else - I think it is possible to BEFOREHAND, trim them down, summarize them into a small mini-summary of themselves, rather than use the full system and just drop parts here and there inconsistently.
But most of these other games, I like better, system-wise in general, because they have fewer rules covering fewer "situations" and eventualities - to my knowledge, D&D and its clones and adherents are the only games to use Attack of Opportunity rules quite the way they're written in those books - others may allow a free attack at a fleeing enemy or something, but I don't know, like I said, I think D20 is just obfuscated to the point of obtuseness, maybe for its own sake, and I just see no reason for that. It's a GAME - I should be able to just sit down and play it - I shouldn't have to decipher it or work trigonometry functions or index Appendix XI for everything that happens. It's just hopelessly overblown, but a lot of other games are that way, ones that are similar to D20, but also ones that are different. And the amusing thing is, and I'm mentioned it before, is Gygax, D&D's creator, the year before he died, said he really should have gone and stayed rules-lite with his stuff, he was sick of how complicated games were. Oh the irony.