It all comes down to your opinion of "Bashing."
If someone is jumping into a thread, saying "Oblivion is a pathetic waste of an RPG", but they don't back up "why," or say anything at all to support it, that is Oblivion-bashing.
If someone is jumping into a thread, saying "Oblivion is a pathetic waste of an RPG," and they then proceed to highlight exactly why they feel Oblivion is said waste of an RPG, then it is a valid opinion.
Could it be argued that they simply dislike parts of Oblivion, and that saying they dislike the game as a whole is being melodramatic? The answer is "No." Someone could certainly dislike parts of Oblivion and yet still enjoy the game as a whole, but there is a vast swath of people for whom the sum of disliked parts outweighs the enjoyment of the game as a whole; these are people who either put down the game and don't return to it, or they have to mod the game like crazy to make it enjoyable. So saying, "This game as a whole is a flawed implementation" is not over-the-top.
As for why this opinion appears all over the place...
Certainly, in some contexts, it might be a bit tactful to put things in a different wording to minimize conflict. But the fact remains that this is a forum board, and it's users have a whole collection of opinions on a wide variety of subjects. In the aforementioned "Applause for Bethesda" thread, if someone disagrees that Bethesda should receive applause for Oblivion because of their opinion of Oblivion, then that's not bashing. That's contributing to the discussion of the thread. Are we going to put up disclaimers on threads, saying "If you disagree with the basic premise of this thread, then don't post" now?
And it is very important to note, as others have, that the possibility of TES:V weighs heavily on all of this. People who strongly disapprove of Oblivion do not want to see what they disliked make a comeback. So they voice their opinion. This can be regarding individual mechanics, true, but it's also regarding the overall design philosophy that permeates Oblivion. The mechanics that people disapprove of are a direct result of that Oblivion design philosophy. And that philosophy is what people reject perhaps most of all.
I must say that, personally, Oblivion as a whole does not satisfy what Daggerfall, Morrowind, or even Redguard satisfied for me. Oblivion feels almost anti-TES, almost foreign to the Series it bears the name of (not counting KotN or SI; those redeem things).
Its implementations of leveled lists and scaling to always keep the player "challenged" is ridiculously flawed for a whole collection of reasons, ranging from the stupor of always having an exact game-determined match for your level, to the fact that said scaling permanently traps many character-archetypes and play-styles permanently behind the curve.
Its character dialogue is so limited as to be laughable.
Its usage of Fame and Infamy is completely and utterly ABHORRENT; it's a good/evil meter in disguise, and that's so untrue to what TES is about that it's not even funny.
Its main quest is rushed and damages the mood and feel of an open-world "do-what-you-want" game from the outset by causing the player to sacrifice immersion if they wish to procrastinate and delay.
Its main quest is also ludicrous; http://www.youtube.com/watch?v=3K5rGUBUnnQ (WARNING -Spoilers in the link).
Its factions try and fail to grasp "epic," just as its MQ does.
Further, its factions are completely and utterly unrelated to one another and don't affect each other at all.
Not to mention that its factions are just off in their own little worlds for the entirety of the Oblivion Crisis.
Its lore as added by solid and actually discussable mediums like books and meaningful dialogue is limited as compared to its predecessors.
Pertaining to lore, the fact remains that
Spoiler the Oblivion Crisis occurs, Martin sacrifices himself and brings down the Avatar of Akatosh and seals the liminal barriers and all
and in terms of in-depth lore and understanding of that event, we still don't know what the [censored] happened. Total
Morrowind's and Redguard's amount of Kirkbridean works were off the charts; both places reeked of foreign power and intrigue and either Yokudan or ALMSIVI badassery; http://www.imperial-library.info/pge/cyrodiil.shtml that could still have been translated via game mechanics, but they were tossed aside in favor of not just an environment retcon but a cultural, social, and economic retcon.
Fast Travel to any location that you've ever been to before is a beta testing mechanic, not a game mechanic; it is a terrible invention.
Oh, and before anyone comes up with an "It's optional" quip, know that Oblivion's Fast Travel undermines other systems like limited travel systems ala Morrowind, and that Oblivion's fast travel system necessitated the removal of carriages-for-hire, boats-for-hire, Mages Guild Guides, and exotic ancient methods of travel, and the Mysticism skill was so gimped by the removal of intervention and mark/recall spells, it's ridiculous. So this "optional" fast travel system is by its nature killing options. Contradiction much?
The list goes on and on. I don't call this bashing. I call this reasons for why I find vanilla Oblivion largely worthy of scorn and unable to sustain my long-term interest