TL;DR: Player has gained new abilities/more power since Beta. Most mobs remained basically the same. Mobs now balanced to fight players with completely different abilities/power than what they actually have. Game is exceedingly easy as a result.

As Minecraft has developed, players have slowly been able to take advantage of new features that make them stronger than before; sprinting, enchantments, health regeneration from food, shields, etc. On its own, this is a good thing, as it gives players more options to work with to overcome obstacles and makes the game more interesting and varied as a whole. The problem is that mobs weren't updated to handle these changes; most mobs are designed to fight players with completely different capabilities than what they are actually fighting.

Although some mobs did receive buffs, none are strong enough to overcome power creep. This does explain why skeletons are much stronger than the other mobs, as they received fairly consistent buffs while others remained about the same difficulty as the buffs were either meaningless or countered by nerfs. 1.9 did nerf players slightly, but the game as a whole is still much much easier than it was in Beta.

The end result of this is that the game has become extremely easy and does not provide much of a survival experience; how can you call it survival when nothing can reasonably kill you? It's fine for players who play the game for building and don't want intrusions, but it makes the game rather boring for players who want some amount of difficult combat to span the breaks between building. For me personally, I end up blazing through the early tiers in a few minutes, spend some time building, get tired of building, and lose interest since there's nothing substantial to fight or loot left. For a game that prides itself on having features for players of any taste, it's rather debilitating for 99% of the combat to be underwhelming.

To fix this, an update should be released that rebalances mobs so that they can adequately respond to the threat that the current player poses. The exact method for doing this obviously doesn't matter, but the update should do the following things:

• Reintroduce the niches that the four common mobs originally fell into (mobile spiders, strong-but-durable zombies, glass cannon creepers) or otherwise give them new roles rather than the current format of just "melee mobs" and "skeletons".

• Increase the damage of mobs besides skeletons and mobs that already deal very high damage across the board in order to properly deal with armor, or otherwise make those mobs more lethal.

• Update mob AI to recognize players damaging them from outside their sight radius.

• Have DoTs prevent health regeneration from a full hunger bar, or otherwise give mobs using DoTs a way of countering the sped up health regen. (Or just revert the health regen buff.)

• Make common mobs that spawn underground more likely to spawn with equipment or other buffs to counter the increased power of higher equipment tiers.

• Keep Easy difficulty as similar to the current game's difficulty as possible, so players who want mobs in their game but don't want to be harrassed constantly by them–such as builders who prefer survival–aren't disenfranchised by the update.

To provide a few examples of how power creep has neutered monsters:

• Spiders in beta versions were threatening primarily due to their speed; if a spider noticed a player, the player would be too slow to outrun the spider and would be forced to fight it or duck into a space that the spider is too large to fit in. With the advent of sprinting, they lost their speed advantage and essentially became a wimpier version of the zombie. They're designed to fight players that aren't capable of sprinting, and die very easily when faced with players that can sprint.

• Creepers are obviously threatening due to the massive damage from their explosions; however, their explosions deal pitiful damage near the edge, and several updates have made it much harder for creepers to land point-blank explosions. Obviously, sprinting is useful for getting out of range, not just in terms of running outside of the blast radius but also for knocking the creeper away with a sprint-hit. Shields also completely negate the explosion regardless of distance (even though it punches through several cubic meters of wood with no problem?), meaning they aren't dangerous even if you get cornered. To top it all off, the AI update to mobs–for whatever reason–removed their ability to move while hissing. The end result is that they lost almost all of their ability to deal damage in direct confrontations and are only capable of killing unwary or unskilled players.

• Mobs that rely on DoTs–cave spiders, blazes, wither skeletons, etc.–were originally threatening as they ignored armor and curing the DoT usually required spending an entire slot on a milk bucket per cure or using cumbersome regenerations to out-heal the damage. Ever since 1.9, the increased regeneration offered from food easily outpaces the damge from DoTs; you're never going to be short on food, so DoTs are now mostly harmless.

• Armor was much weaker in earlier versions. Although all armor technically provided the same amount of protection as diamond armor does now, their efficacy would decrease as they lost durability and would quickly turn into a mild boost. Additionally, since food was unstackable, you could only have so much healing in your inventory (even if it was instant) before you'd be dead in the water. Mobs therefore didn't deal too much damage as protection bonuses were unreliable and healing items were cumbersome. In current versions, armor always provides a certain level of protection regardless of durability, making iron and diamond armor excessively powerful against most mobs. Food, although it regenerates health over time rather than instantly, is stackable and provides tons of healing in a single inventory space. As a result, most mobs deal inconsequential damage that is trivial to heal unless they happen to be one of the mobs that actually deals a large amount of damage (vindicators, pigzombies, point-blank creepers) or have an armor-piercing component (guardians, technically DoTs but those are nullified by regen).

• Mob AI as a whole is balanced to deal with players in short ranges; mobs don't react to anything outside their sight radius (for most mobs this is 15 blocks). Naturally, this includes arrows. Pre-beta, this was somewhat acceptable as bows worked differently; they were a lot faster, did a lot less damage, and had a lot less range. Fighting mobs outside their sight radius was a slow endeavor that wasn't much more worthwhile than charging in with the sword. With the current long-range bows, however, it's much easier to do so and makes fighting any mob except ghasts, guardians, and blazes (and technically zombies) completely risk-free. it quickly becomes egregious for endgame mobs like shulkers and vindicators to do absolutely nothing when shot and turns their dungeons into cakewalks (mind you, bows are an early-game item…).

• Most mobs are balanced around stone-tier players, since they were either created when the difference between tiers wasn't great (before armor changes) or because they were created with the balance of those earlier mobs in mind. Back then The rift in tiers created by armor does a lot to widen the gap, but enchantments–effectively a tier above diamond–makes those mobs even more hopelessly incompetent. They can't break through diamond's protection, and can barely touch iron; they'll be useless against enchanted diamond. Offensively, mobs have trouble dealing with Sharpness V swords, but are even more useless against the 23-damage Power V sniper bows than they are against the regular ones. Only Guardians plus the two bosses have the stats to properly deal with enchanted players. (Shulkers don't deal enough damage and Illagers are handily taken out by bows anyway.)

End result is, three of the four common mobs (which you fight 99% of the time) got nerfed in particular, while everything else that existed in Beta never got updated to handle all the shiny new toys that players got. Future mobs were not quite as affected as Beta's mobs, but were usually balanced with the original mobs in mind and as such fall prey to the same balance issues.

The sole exceptions to the power creep are the Wither (statistically too strong to be killed by anything but the strongest gear, not affected by the bow issue due to sight radius/arrow shield at half health), the post-1.9 Enderdragon (deals decent damage and is designed both with bows in mind), and guardians (water prevents bow cheese, has high damage that partially ignores armor, more mobile than players without Depth Strider).