The title is pretty self-explanatory, so I'll dive right in.

1) Involvement in the Thief and Assassin guilds: I'm one of those apparently rare persons who derives no pleasure from criminal activity, even in a purely fictitious game setting. Those guilds therefore do not interest me in the least. Now, if the vanilla game had been more sophisticated ,or some ambitious modder took up the cause, my avatar would have willingly join either of those two organizations AS AN INFILTRATOR for, say, the Elder Council intent on bringing scum to justice.

2) The Adoring Fan: I remember first seeing web discussion of the Fan and thinking "Whoa... what is this? A mod? I've certainly experienced nothing like it in MY game!". As it turns out, I never met the fan because my avatar never finished the quest line that spawns him. During my first play-through, I abandoned the quest at its very end. Upon learning what must be done to earn its ultimate reward, I realized I didn't have the heart to do it, so simply walked away and never returned. This play-through, I totally forgot about the quest-line until too late for it to make sense for my avatar's to participate. Just as well really, since my stance on the final act hasn't changed.

-Decrepit-

I've never completed the dark brotherhood. With my very first character she murdered someone by accident and started doing the quests for it to see what it was all about, but I never got very far with it, I just didn't like it at all.

I've never done any master training either, though I know there's mini quests associated with them. My skills just never go high enough for that to happen.

Oh, also Marksman, I never have that as a major skill. I just can't use a bow, I find it too difficult. Pretty tragic really.

I have never done the master training quests.
I've done all the guilds, but the last one I did was Fighter's Guild, because it is the absolute worst quest line in the entire game.
Never been to Dive Rock, either.

Lothran has never experienced how it is to be working for the empire. He never handed over the amulet to Jauffre even. He has never killed on command (DB) or become the head of any guild

Boringly, I have done all the quests with various characters. For the past three years however, my only character has been Buffy. Despite her longevity, she has done few quests, favoring exploration and clearing non-quest dungeons.

She has done few of the Daedric Shrine quests. She is very active in her guild of mages, but will never rise very far in her guild - she is certainly not Arch Mage material and unbeknown to her, she will never see the destruction of the Bruma Guild. Losing Sigrid's guild at Kvatch was hard enough.

She does do quite a few quests that are either not represented in the game or that she twists until almost unrecognizable. For example, she did fairly well in this year's archery tournament held in Bravil (Alawen won). She talked the Gray Prince into leaving the Arena; he is now the consort to Sir Mazoga and a Knight of the White Stallion.

The Main Quest is happening around her, but at a glacial pace and she only plays a small, supporting role.

It would be just her style, that if she ever got involved with the Thieves Guild, that it would be purely to indirectly support a storyline that involves helping Countess Millona Umbranox finally resolve what happened to her missing husband. Another example of how Buffy might handle a quest is to investigate the missing Lenwin, disarm her of the blade called Umbra. Then destroy the evil blade in a lava flow to free Lenwin's spirit so the sister wood elf could peacefully return to her life at Pells Gate. . . .

Although we do run some mods, the changes in her game from vanilla are almost entirely imagination, not mod induced.

She is quick to wrinkle her nose at many quest requests because they don't fit her eclectic sensibilities. I suppose, therein lies the benefit that I have seen all the quests completed at one point or another. I don't mind a bit if she refuses many.

This. I don't think there is anything I haven't done in Oblivion (except maybe collecting every Nirnroot, which I don't intend to do anyway.) It's possible there's a dungeon or two I have never been in, but that's about it.

As far as what Gwendaline Balois has never experienced, it's quite a lot. She's only joined the Fighter's Guild and has only risen in rank to Swordsman. She has not done any Daedric Shrine quests and I'm unsure if she ever will. Most represent things that go against her morality, and one of the only Daedra that she might be mildly in support of, Hircine, has a quest that I'm certain would upset her companion, Vilja.

The main quest is relevant to Gwendaline's adventures, but it is not going to be completed by her. I typically play multiple characters and, canonically according to me, all of my characters exist in the game-world roughly around the same time. Therefor, the Oblivion Crisis is happening in Cyrodiil as I play through Gwendaline, but it is actually my Imperial woman, Nona Sintalius who is completing the main quest. Gwen just sees Oblivion Gates popping up everywhere.

She's also done a few side quests, like helping the Forlorn Watchman or making the people of Aleswell visible again.

Typically, the first time through the game, I do everything possible with one character, then I start making other characters designed to do certain things in the world from a much more hardcoe roleplaying perspective. All of my characters do something in the world, but they never do everything.

I've never done the Dark Brotherhood, nor have I completed the daedric quests for Molag Bal, Hircine, or Mephala. I find that these quests demand actions that I (and my characters) simply won't do. There are a couple of other daedric quests that are only done by insane or desperate characters (Sheogorath's "prank" comes to mind.)

I've finished the SI main quest exactly once, and will not finish it again, although my characters often start it. I consider the SI main quest line to be fatally flawed from a role playing perspective, since in several places the plot line is linear and the player is forced to choose between two equally unsavory actions, even when a third (obvious but unavailable) choice comes to mind. Worse yet, there are a couple of sub-quests where there are no alternatives at all, but only a linear do-this-do-that sequence. And worst of all, the player cannot make the obvious choice to join with the "other side" in favor of Order.

In 4 years of playing I've never played as a vampire. Never done most of the Daedric prince quests, and have only once done the DB Thieved guild and SI main quest.

I've also never use a blunt weapon or a staff. I've never done a master training quest.

My current character, who I've been playing for 8 months now, has never cast a destruction spell, never enchanted an item, never trapped a soul, never received any skills training and has never fast travelled. She relies on archery, alchemy and illusion spells.

One of my current characters ("Wild Elf") has never set foot inside a city, and has never done a single quest. She's primarily a mage (a "Witch" actually), so lack of access to spell vendors would make her life impossible, were it not for the fact that we are running the Spell Tomes DLC. The random nature of the tomes we find makes the game very interesting.

Throughout Daggefall, Morrowind and now Oblivion, I have never had an assassin. Until recently, that could also be said for the Thieves guild but during her childhood, Angel, being the prankster that she is, liked to peek in peoples pockets to see what they were carrying. Of course, it got her in trouble. She is a little anti-authority, Believing that guards are there to protect and not lord it over everyone and since she had done no real harm, she joined the thieves guild to bribe her way out of trouble.

In Daggerfall it was possible to provoke the Assassins guild and they would send their minions to attack you at random times. Some of my characters got very rich looting failed assassins. It's interesting that the city guards would walk right past an attacking assassin but don't you dare attack a citizen yourself.

Severus Snape entered the arena twice. As he left the second match, he angrily declared that he was no butcher and would never again slay anyone for entertainment. Sarrah entered one match and drew the same conclusion. Angel has never entered the arena and never will; neither to fight or to gamble.

Both Severus and Sarrah joined the fighters guild and ended their association with it when they were spoken to very disrespectfully by Modryn Oreyn. They both resented his sneering tone of voice, walked out and that was the end of that.

From Sarrah, Angel knows the fate that awaits the Bruma Mages guild and so she refuses to seek a Mages guild rank above Conjuror. In fact, she is extremely proud of her rank and has no wish to be Arch Mage.

I've never done the Hermaeus Mora quest. No one character has ever done all the other Daedric quests, although I've done each of them with different characters. There are quite a few Master Trainer quests left undone, too.