Did you miss the part where I said that games are a machine of interconnected mechanics?
One mechanic alone does not make a game automatically better, it depends on how it works with other mechanics and the rest of the game. Serious Sam wouldn't work with a two-weapon limit because then you wouldn't have enough weapons and ammo to deal with the enemies the game throws at you. However, some people naturally assume there HAS to be a two-weapon limit because it is realistic, with little regard for cohesion. A good example of a two-weapon limit is Halo, which is actually balanced around that limit as is explained by this frog in embed related. For most shooters, such a limit is usually thoughtlessly put in.
A mechanic by itself can't really be considered shit or good, it all depends on the implementation. Mechanics that are often badly implemented like regen health and level scaling get a bad rep because of that. There are some mechanics which people like like canceling and juggling, but as always, it depends on execution. Games can be (and are usually) built around limits, which is nowhere more apparent than in Castlevania and Castlevania 4. Castlevania 4 gave more freedom in the direction of your attack, and ended up making the game (too) easy. The subweapons were meant to offset your limited attack range, and through clever subweapon placement a clever player could turn the situation to his advantage. A Medusa Head wouldn't be a problem to deal with in Mario, in CV it's a fucking cunt to deal with. Point being, that you can have limits in your game, as long as the game is balanced around it.
You can also have the blandest and most unoriginal gameplay concept, which may still be good if you execute it just right. Thunder Force 4 looks bland on paper, but is a blast to play.
There's more competition, so people are more willing to follow/copy trends instead of standing out? I don't think that's how it works.
Concerning the usage of AI in games, consider the following: All zombies usually do is crawl towards you and try claw your face off, their pathfinding is capable enough to follow you around, but it doesn't really do anything but follow you and mug you from up close because it is a zombie. Does the lack of intelligence present in a zombie mean it has poor AI? If so, what would a zombie with good AI even look like?
Sometimes, an enemy doesn't need 'good' AI if it accomplishes in role in combat just fine. A better question, how are you even supposed to notice the effects of good individual AI when you're fighting dozens of various enemies at a time? What would Doom have to improve on in order for its AI to be considered good? You're making Carmack angry here.
You'd have a point if you were fighting the same enemy type most of the time where everyone would behave the fucking same. In a game like FEAR, the AI is what helps makes the combat feel unique and different from other encounters since you usually fight a bunch of identical replicas, whereas in Doom it's the enemy placement and enemies used that make each encounters feel different, by using different enemy types in different ways. If an enemy accomplishes its role on the battlefield (trying to kill you by doing X) then what's there to complain about poor AI? That's not even considering the limited processing power at the time of Doom's release and how demanding it would be for the CPU if it tried to compute dozens of enemies with complicated AI subroutines.