I'll pull out my PR essay that Erazor posted for me there:-
Considering the arrival of the base stats of all the fully evolved Pokemon(which has been confirmed by Serebii), this is when we need to realize that we cannot use the same power standards as we did for DP. There are a lot of Pokemon that have at least one base stat about 105(just an arbitrary large number), meaning that they(at least from a theorymon aspect) are going to cause large ripples in the metagame. If we expect to start with some predefined bans, we are at once beginning with a mindset that Generation 5’s OU will have the same approximate power level as Generation 4’s OU. This is a fallacy, as while we may achieve a balanced metagame, we are then trying to achieve a V2 DP metagame, which while keeping us in our comfort zone, is likely to cause an extremely large ban list of new Pokemon, due to them being above our perceived “required power level”.
There is an extremely good chance that some of our current Ubers may not be so in Generation 5, due to the addition of several Pokemon to the metagame. Hypothetically, if say, Deoxys-A was balanced in this metagame due to a lot of priority users being available, pre-defining Deoxys-A to be Uber would be a mistake, since it does not overcentralize the new metagame. This would lead to arguments and debates later, when it could have been easily solved earlier. Note, this is just an example.
While this may be argued to be an “Ubers Lite”, let us examine what we mean by Ubers Lite. We mean that the power level will be lesser than that of Ubers, but larger than that of OU. However, another fundamental concept which we need to understand is that the so called “Uber Lite” power of one generation may be the “OU power level” of the next.
OU is decided by usage, but also indirectly by power, due to anything being perceived to be overly powerful and centralizing being banned to Ubers. Thus, OU has a certain power level, and it has to be accepted that the level changes between generations. Trying to set pre-defined limits on OU’s power range based on a previous generation would be an appeal to tradition, trying to make ourselves comfortable in a metagame we are used to, rather than trying to experiment with a new one.
So basically, starting with no bans for a 2-week testing period would solve a lot of these problems. We can run such a testing period by opening a thread where people can report what the new metagame is like, what is powerful, what was hyped incorrectly etc. If there is too much outcry about particular Pokemon being broken, we can test those specific Pokemon with whatever testing process we intend to use then.
If anybody wants to say that people would be discouraged from entering an “unbalanced “ metagame, I see no need to advertise a “balanced” metagame while we are in the process of testing at the beginning.]
tl;dr: Trying to have a pre-defined banlist for Gen 5 is like trying to preserve the DP metagame in a slightly modified form, which is not the aim of our tiers system. The real aim is to provide a competitive metagame, which does not have to be very similar to the previous generation’s metagame.
Yeah, it sums up why I feel we should start with no bans. I do support Species Clause, Evasion Clause and OHKO clause imposed at the starting though, but I guess that's a given.