HI
I think the reason behind why Oblivion isn't considered an RPG is this:
You can do whatever you want, it is free form.
This means you need to PLAY it. Not GAME it.
And by PLAY it, I mean the word play usually is connected to loosely unplanned activities in which you have a great say in how to these should get planned and implemented (carried out). Children playing outside on a hot summer's day is an an example of this.
By GAME, I mean the word 'game' usually is used about activities in which there are strict rules which must be adhered at all times etc. etc. Think of a boardgame i.e. Monopoly which has rules etc. etc.
And since the first Crpgs was very likely connected to or tied to the D&D rules, people have grown to expect that all Crpgs should use the D&D rules.
(and this is like the boardgame, I mentioned)
Then, Oblivion etc. comes along and makes it so that there no rules (or a few rules) to follow, as well as the player can make the rules up as he go along (sort of). Then people start to say 'but it is not an rpg'.
My answer would be: 'yes, it is an rpg, not just the rpg, you were expecting'.
I have two major grievances with Oblivion
1) the mini-games in which it is the player's skills with his fingers that decided the outcome of the mini-game, not the stats and skills the player's protagonist has.
2)
The annoying pop-up messages which guides you through the game as if you were a 5 or 6 year old.
However, these grievances do not make me think that Oblivion is not an rpg.
aries202