Continuing the theme from last update, I'll be going over which genres should and should not employ some sort of difficulty system. In the case of the latter, I'll suggest what alternatives they could use (if most of them don't already).
1. FPS/TPS - Essentially for reasons the blog (and elsewhere) has gone over already: Tactics vary at higher difficulties. Whereas you can charge through most areas on Normal like an action hero, higher difficulties will require more cautiousness, a deep understanding of the level you're on (regarding where enemies and items appear, which is why a previous Normal playthrough is beneficial) and some degree of stealth. In certain types of shooter, ammo conservation is also important for the harder modes, since bosses often require an awful lot of it.
2. Survival Horror - Like mentioned last post, Survival Horror uses a difficulty system to basically save people from having their spooky experience too bogged down with fighting and bosses. Ideally, a low difficulty setting will make the same number of enemies show up but make them far less aggressive, heightening the creepy tension by all the weird monsters stalking around without forcing you to destroy them all to progress. Of course, a good deal of survival horror requires that you don't fight the enemies anyway - Siren and Clock Tower are examples of this sort. Perhaps in those cases, where you're not expected to fight, the difficulty could be raised for additional challenge in getting past them.
3. Stealth - Similar to the above, a modern stealth game usually offers players two ways to progress: to make as much noise as possible and defeat the enemies standing between you and your objective, or to sneak past without a whisper. Ideally, there would be a slider bar of difficulty, with "Stealth" on one end and "Action" on the other. A Stealth-oriented game would increase the enemy force's deadliness (perhaps one-hit kills under almost any circumstance) and drop their attentiveness, making stealth a viable option. Inversely, setting the bar far along the Action side could make stealth very difficult but enemies far more accommodating to dying from gunshot wounds to the face.
4. SRPGs - This only applies to the "chain of story battles" SRPG, like Fire Emblem or Vandal Hearts, instead of the more open and less specific battles of the Nippon Ichi SRPGs like Disgaea. The best way to incorporate a higher difficulty setting in these games isn't (only) artificially raising the stats of enemy combatants, but to insert additional battles throughout the story that are far more difficult than those found in the normal setting. This can be done either as a post-game set of "challenge maps", that might provide backstory or a previously non-player character's viewpoint to some scenes earlier in the plot, or it could be on a completely new playthrough with the extra battles inserted as and when they occur in the story.
1. RPGs - The difficulty in these games are usually self-imposed: You could decide to fight the Ogre at a low level, or simply avoid him for now and come back later. This is a little more applicable in Western RPGs (which tend to be less linear) than JRPGs, though with the latter there is always the option for any player to attack the next boss along the story route as they are, or to spend a few minutes grinding in the current area. RPGs work because players go at their own pace, and fighting a boss under-leveled is much more rewarding for any expert player than to simply let the CPU boost the enemy's stats, as it is a self-imposed challenge. A decent alternate is the New Game+. In this instance, enemies are upgraded (or often not) and the player is allowed to keep some or all of their equipment, levels, cash or anything else they've earned on their first playthrough. This assists with achieving 100% on a game (especially one with branching paths that makes 100% impossible for a single playthrough) and the game can also unlock some difficult side-missions, dungeons and bosses for the second time through. Players are once again advised to go at their own pace.
2. Roguelikes - Roguelikes don't require a difficulty curve, because they're largely random. Of course, it wouldn't do to put the hardest monsters and best equipment on the early levels (unless there was a way of evading the first and enforcing a minimum level limit on the second) but in most situations, a Roguelike's difficulty is defined largely by luck.
3. RTS - This is a purely subjective standpoint (as opposed to the others, which are only mostly subjective) but I believe RTS games work best when they provide players with multiple factions to control. This is pretty much true of any modern RTS of course, but when relating these factions to the game's difficulty is where things might get mildly controversial. Ideally, each campaign has its own difficulty setting - this way, you have a difficulty curve to progress through and each faction's campaign is different enough to warrant a playthrough of each based on originality alone. There are a few problems with this: the "easy" campaign might move too slowly for experts (maybe shortening that campaign and making it more of a "tutorial" of sorts might fix that) or maybe that someone's personal favorite faction might also be the hardest, forcing them to learn the ropes with a group they didn't want to play as. Nonetheless, this is a good way of incorporating a difficulty mode of sorts without forcing the players to play through the same campaign multiple times with a slightly tougher opponent.
4. Platformers - Kind of obvious, this one. A platformer's difficulty is inherent in its level design - to increase the difficulty would be to rebuild the entire game world. The more shooter-based platformers (like Ratchet and Clank) are exceptions though, provided the difficulty is based on the shooting part.
5. Sandboxes - Really, this includes any large, non-linear game (most of which are now known as sandboxes). Generally, you have the story missions - which are of a casual difficulty - and as many harder, optional missions as you'd like to take on. Another reason why difficulty shouldn't apply to sandboxes are their size: After fully exploring some five square miles of real estate for hours of gameplay, why would you want to go through all of that again on a slightly harder setting? Better they follow inFamous' example and have the second playthrough be subtly different based on something like an opposing morality (though that game has a redundant difficulty setting too, if I recall).
6. Sim Games - I'm a proponent of variable difficulty for games like the Sims, or any game where you assume the role of some developer or creator. If you're doing well the game should create challenges for you, but if you're struggling they should hold off on dumping extra trouble on you and offer help instead. Any other type of simulation game (say, flying a plane) should only be as difficult as the real life activity you're simulating, since realism counts.