Fact: Mike Mearls is a mammal.
Fact: Ninjas are mammals.
Fact: Mike Mearls is a ninja who eats sacred cows.
.....Sorry, I just had to reference that thing 'cause it's all I could think. *Ahem* So, I would argue that 4e drops in number of sheer options for developed Learnability, Flexibility, Robustness, Consistency, and effectively limiting the power of constraints.
Learnability - The ease w/ which a new user can begin effective interaction.
Flexibility - The multiplicity of ways in which the user & system exchange information. In this, I mean that the intentional modularity allows the system to be easily altered to suit one's needs.
Robustness - The level of support for user determining successful achievement. While this can have several uses, I will limit it by applying it to the way in which it is much easier to create a character who is functional in play.
Consistency - The power, race, and feat rules (barring some errata and anomalies) show remarkable consistency.
The Power of Constraints - This refers to limiting the possible interactions to key the user into what interactions are meaningful.
And that last one is the clincher. Some people do not like being given advice, even if it is good advice. The game's limitations and constraints - while designed to engineer meaningful gameplay - feel like wing clippings to some people. While not all bad options were removed (Sure Strike

), most of the game consists of 'good options'.
(This of course leads to the flawed assumption of flawed characters being better for roleplay... which somebody argued against quite well, but I do not recall that exact argument. Suffice it to say that a player can still create a dynamic character even if they are actually good at useful things)
Edit: This relates to the OP in that the lesser [fought] monsters would be relatively easily created for situations, and if the monster was to be an ally/not fought... it could get NPC rules. This goes back to the first three, and constraints would lead to the monster being constructible from synthesizing the objective and the monster creation rules.
...now I'll go back to professing my love of Tempest Fighters, etc. Thankyou.