Some of the monsters are more scary than others. For example there are some caves in Act 1 which are very dark, gloomy and claustrophobic. They are inhabited primarily by some sort of slimy squids which are pretty disgusting the first time you encounter them. After a while you get used to them but sometimes you still get the creeps when you see them slithering towards you from the darkness.
The AI for these monsters is nothing out of the ordinary. I did notice that a lot of ranged mobs (such as humanoids or skeletons firing bows, or giant cobras shooting poison) would attempt to run away out of melee range when wounded. This did create a bit of a problem for my melee character who had to run around to catch them, often failing to hit until they decided to stop.
From my experience so far, it looks like Path of Exile follows Diablo 2's mob system very faithfully.
The majority of monsters are normal, with white nameplates.
Occasionally, just like in Diablo 2, there are groups of champion monsters, with blue nameplates. Almost without exception, you will encounter groups of these monsters. What differs from Diablo 2 is that if the game happens to generate several different types of monster in close proximity, and then makes some of them champions, you will get champions of mixed types. For example you might get a group of champions consisting of spiders, bandits and skeletons, accompanied by normal mobs of those types.
Champions differ from normal monsters first by the larger health pool, about 2x-3x as much as a normal monster, and second by an extra attribute. The screenshot below shows a champion with the "Moves Quickly" attribute. Champions always drop an item and quite often that item is blue (magic).
The third type of monster, once again like in Diablo 2, is the rare boss. These have, you've guessed it, yellow nameplates. They differ from normal monsters by the much larger health pool (at least 3x larger) and by the extra random abilities and attributes they may have. The one below has 3 extra attributes.
Rare bosses most often have auras that they share with their minions. The aura for the one in the screenshot below gives the effect "Allies Have Increased Accuracy and Critical Strike Chance".
Here's another rare boss. Notice how dark and dreary that dungeon is.
Finally, there are unique bosses whose nameplates are dark gold. These are always the objects of a quest. They are vastly more powerful than a normal mob (both in health and damage) and they also have a variety of abilities which are not listed under their names. Some of them are accompanied by minions. Here's a couple of unique bosses.
As you can see, Path of Exile's monsters are very Diabloesque in spirit but also much better looking, as well as some of them being downright scary. If you've been following Diablo 3's development, you might notice a difference between these two games. Path of Exile looks more serious and grim, while Diablo 3 has lightened up on the atmosphere as to appeal a lot more to casuals. Either way, both games are fun in their own way!