I think you missed my point--this thread is about combat. Considerations such as negotiation or stealth are irrelevant. My point is that reducing PC specialization frees up the DM a great deal in potential adventure design, and this holds true for combat, exploration and social situations.
Of course you can argue that in doing so every character ends up looking the same (and that is one of the primary complaints about 4th edition), but I never felt that argument held any water.
Combat, Exploration, Diplomacy - it doesn't really matter. The campaign will be more fun if you cater to your players.
Even if it's purely combat, and
even in 4e (which is probably the most easily balanced since as the OP points out there's little variation in what effects what), you'll have PC's that are good against solo's and PC's that are good against hordes of minions. Regardless of your personal opinion of solo's and minions, the PC with his huge bursts will appreciate it if you introduce a horde of minions now and then; whereas the solo executioner will appreciate it if you let him run roughshod over a solo now and then. And irrespective of their strengths, different players want different things from the game, which it's helpful to cater too as well.
This is no different whether its combat, exploration or diplomacy: catering to players (or at least
considering them) is part of DM-ing.
I don't want a return of 3e's mechanics for favored enemy - I'm not disagreeing with you here. I'm objecting to the tired, unhelpful tip to be fun and engaging. Obviously a game should be fun and engaging; saying it doesn't make it happen. The advice is way too abstract; as is demonstrated by the subsequent advice to not cater to your players. And it certainly isn't an argument to only have "effect everything" abilities.
How about damage types with their accompanying resistances/vulnerabilities? Those are an example of rules for effects that are somewhat specific. There's been rumors of a possible revival of weapon-damage types (e.g. bludgeoning vs. slashing etc.), do you think that's a good idea?
I think that game rules that encourage adapting your strategy to the task are a good thing; so I think that the general idea of moving away from "effect everything" abilities is good but the OP is looking in the wrong place. I'd much rather see limits to things like expertise (4e), weapon focus (3e/4e), weapon specialization (3e) and magic weapon bonuses so that PC's aren't hyper-specialized to the point that they
need to keep using the same tool.