SIMPLE GAME RULES.
Most importantly, D&D and other RPG's are, well, games. The rules should be just that, simple rules by which players and DMs know where they stand and can interact in a gamelike fashion (particularly in regard to combat and the like). The old playability vs. realism debate impinges upon this point, but in many current TSR releases the balance has swung so far to the latter that rulebooks read more like discussions on medieval topics than rules for play. Take, for example, the Companion Set "Dominion Confidence" rules that don't do any more than suggest that such morale levels can possibly rise or fall at a DM's wish. Or the "Champions of Mystara" set which is dominated by a storybook about someone else's adventure, and a general discussion on economics which contains about two or three actual numerical suggestions in thirty or so pages. Tacking on rules systems in odd supplements, with no general core agreement (such as whenever one wants a fleet of flying ships or for low-level characters to travel the planes) contributes to this problem. So does the baffling idea that in-character narrative passages are a good way to express information on settings and rules (starting with the old "The Ecology Of..." articles and now infecting every boxed set and booklet of TSR's), when clearly this device only obfuscates and makes information hard to find while a game is in progress. Apparently TSR's current readers have such a short attention span that they can't sit down and read concise rules; gone forever are the days when EGG would clearly explain the rationale of why such-and-such a rule had been devised, and how a DM could utilize it.
One of the worst examples of un-gamelike rules was the "Wilderness Survival Guide" (and in fact, most other weather generation systems). Great deals of meteorological information, how to go camping, what temperatures are high and low for any day, and so forth, are given, with little or utterly unwieldy information on how it would bear in game terms on characters. Movement rates given in miles, or yards per five minutes, and fatigue penalties in half-hour intervals, are all crushing game-spoilers, when players want to be able to move in daily or weekly units of time. Ideally, as in a boardgame, movement rates should be given in spaces-per-turn, whatever those measurements be, and players can then move figures or the like on a map and not be calculating excess or remainder mileage or fractions thereof. Even in EGG's new "Mythus" fantasy game the same problem occurs, where he gives ranges in degrees Fahrenheit for comfort in various types of clothing, with little direction as to what to do when those limits are exceeded, or how to determine the current temperature or how it changes. (To make these rules gamelike what is needed is some kind of determination if a given day is: hot/cool/cold, etc., in a handful of ranges, simple standards of clothing for each range, and a few dice damage if the PC is inadequately clothed. Compare this to the WSG system which gives: a dice roll for daily high/low temperatures and wind range... DM determination on an hourly basis as to exactly what the current temperature is... player calculation of "personal temperature", adding points for clothing (vs. current temperature), a "protected"/"unprotected" status determination, checking a wind chill chart, checking a humidity chart, adding points for armor, adding points for exercise or rest in five-minute intervals, campfire effects applied in 5-foot distance increments, etc., etc., culminating in cross-checking a temperature chart for actual effects: this, probably varying for each character and each gust of wind. To admit it, this system killed one AD&D campaign I had, when I tried quixotically to use the Wilderness Survival Guide system).
One of the strengths of D&D was its "class" system of professions, templates detailing character types and relative proficiency. On the one hand, players coming into the game were generally given exactly what the game promised: do you want to be a warrior or a wizard? A human or a hobbit? -- and this was exactly the class they received. In addition, it made detailing the hordes of NPCs used by the DM an easy task: consider when, in the text, we read that we have encountered a "C5". Immediately we know the character's profession, have a range for hit points, weapons usable, "to-hit" probabilities, saving throws, special abilities, and so forth. When D&D and other games (such as "Mythus") introduce chapters full of skills and relative proficiencies (as most games do nowadays), purchasable by characters, it certainly makes the PCs more personalized and specialized and interesting. However, it makes detailing NPCs a nightmare for the DM, as these skills really need to be detailed for everyone: notice in the "Mythus" and Paranoia game systems that it usually takes a whole page to detail any one non-player character's statistics (mostly this is all of their skills). The elegance and efficiency at the core of the original D&D rules and miniatures battles is lost.