This topic is near and dear to my heart. I really should have posted earlier.
I think the qualities that define an Über were very well laid out at the beginning of this thread. The problem is that none of those qualities is measurable. It's all subject to opinion. Hence, it is overwhelmingly unlikely that an agreement will ever be reached on the subject.
There are a lot of people here that are arguing what should and should not be allowed in the 'standard' game. I'm going to try to look past these arguments to base motivations (I could be way off-base here, so yell at me if you don't fit into one of these categories).
1. People who want specific (favorite) Pokémon to be viable in the 'standard' tier so that they can have battles against many opponents and still use said Pokémon. This goes both ways. There are people who want 'UU' Pokémon to be more viable, but there are also those who feel the same way about Pokémon like Salamence, Tyranitar and Garchomp, even though a good argument can be made that they severely centralize the metagame.
2. People who want to have the most viable options in the standard metagame. These people want to have a set of banned Pokémon that will maximize the number of Pokémon that can be used effectively in standard play. I'm mostly in this camp, but I recognize that finding this 'maximal' set of Pokémon may not be possible.
3. Traditionalists who like the current metagame and see no reason to jump to a new one before the current ban list from RSE is adequately tested first.
I don't think any of these positions is better or worse than any other, but it's entirely possible that they cannot all be met in the same play environment. I think I have a possible solution.
I propose two basic play environments. The first environment would allow all Pokémon, including those that are considered Übers with respect to the standard game (with the possible exception of Arceus). When all of the D/P Pokémon were first revealed, I was infuriated that there were so many new legendaries. But then I realized that with all these powerful new Pokémon, GameFreak has created the possiblility of an "Über" tier that is actually varied and interesting.
The second environment would be like the one that I have been proposing (shameless, I know, but bear with me). It includes no legendaries or Pokémon with base-600 stats. Many of these Pokémon have been allowed for a long time, but that doesn't mean that they don't severely restrict the number of viable Pokémon in a tier that includes them. As long as both of these environments have enough supporters, they can co-exist. People who love their CSMence and their Tyranitar, I believe that these Pokémon are viable in the first environment, but they will no longer rule the roost. Those that want to use some 'UU' Pokémon, most will be viable in the second environment. Neither has to be considered 'standard'.
I believe this solves the main Über problems, but it still leaves out many Pokémon that are 'outclassed' by other, non-Über Pokémon. I reference Swampert (which outclasses Gastrodon and Quagsire) and Staraptor (which outclasses Pidgeot, Fearow, Dodrio, etc.) as two examples. I am against banning Pokémon for this reason. For one thing, it opens up more subjectivity about what does and does not outclass what. More importantly, there is an environment that is perfect for these Pokémon, and that environment is 2v2.
Pidgeot gets Tailwind: a practically useless move in 1v1, it's power is incredible in 2v2 with the right Pokémon on your team. Dodrio gets Acupressure: you can target your ally with this. Mixed sweepers that aren't quite fast enough (like Blaziken) love being hit with Acupressure because they can greatly benefit from having any of their seven stats raised. Fearow gets Assurance: a useless move in 1v1, but powerful in 2v2 with the right ally.
Likewise, Quagsire gets Water Absorb (great for when your ally has Surf) and Gastrodon gets Storm Drain along with Counter and Mirror Coat (draw targeted water attacks and KO the attacker).
It's clear to me that GameFreak worked hard to make 2v2 an incredibly varied and interesting environment, and I strongly recommend taking advantage of it. Trust me, Double Battles are a ton of fun.
Anyway, sorry if I appear to be a little off-topic, but I strongly believe that 'what makes an Über' can only be discussed in the context of a tier system. The two really can't be separated.