I said it a moment ago and I'll say it again. Pen and paper RPGs, the genesis point of the genre, were based on (and showed) numbers and detail. It wasn't a longsword +< medium increase>, it wasn't 5/GUESS damage reduction; it was a longsword +2 and a 5/+1 damage reduction.
You may hate on MMO's all you wish - but numbers were in RPG's long before even the original warcraft RTS was a spark in the developers mind.
I dont hate MMO's as i tend to play WoW myself from time to time,but that is WoW and this is TES.
No matter how any other RPG worked,THIS isnt any of those games.
For that matter,almost every other RPG also used Classes,something they deliberatly left out of this particular game,to open up more options for the player to go as they progress through the game.
Yes,RPG's have always used numbers,Skyrim still does so too,but in a very diminished form,so that it does not
control the gameplay.
Micromanagement of a game is good and all if its a competitive game (like WoW),but it has very little value in a pure single player game like Skyrim.
Also,as stated before,Beth went heavilly on Immersion for this game,one of the biggest points of interest before it came out.
That explains the lack of precise detail concerning numbers.
Edit:
You do have a point with the Illusion example.
But i do believe that Vermin and 'lower' creatures still maintain a certain degree of lvl.
To be more specific,i dont believe this lvl points towards an anemy as for example a lvl 5 or lvl 15 Draugr,but more as a Mudcrab being a lvl 3 and a Sabrecat being a lvl 15.Creatures and enemies do scale,but their lvl can be more compared to an 'enemy type',so a certain lvl spell will only work on specific 'enemy type lvls'.
This is pure speculation,i havent tried out comparing the same skill lvl spell on say,a Sabrecat at lvl 5 and a Mudcrab at lvl 30.