This could be intended but it doesn't make the game more difficult, just more annoying.
Now you the time available for exploring is less than a half than before.
This change really breaks the play style of the majority of the players. Omitting that you can't explore anymore over night unarmored or with light armor, when you go out from home you find zombie hordes and lots of skeletons. In swamp biomes, a lot of them survive in the water. And if you don't have a bow or arrows (which is really frequent in the first days of the game) and you find yourself into the water among with a skeleton, you have really little hope to survive and then get back your items.
Exploring caves always result in an infinite (and I'm not exaggerating!) number of zombies spawning and tracking you. You just CAN'T get rid of them.
There is really the need of a major rebalancing of the game.
It is ok to buff mobs, but this isn't really the way to do it: they are just more annoying and the game is getting really frustrating, it isn't funny to play anymore.
Well, you are one of the few.
Search a bit on the Minecraft Forums. Most of users are annoyed.
And btw, this also happens on easy, just scaled down, and peaceful is just too easy, there's no fun at all.
This should be a configurable feature, when you create the world.
You can't just radically change most of the mob AI in one version.
Just an example: http://www.minecraftforum.net/topic/1872524-minecraft-16-zombie-aggro-range/
This covers a lot of interesting points. In particular, post #22 looks like a good summary of all of them.
Mojang seems to care a lot about players' feedback, that is why I'm trying to point out this annoyance.