[Someone needs to take ownership of this proposal. The current description is not satisfactory and is too vague. Please tell us what you are proposing we do with BU. Is this for full nodes to use BU, or for mining nodes to use BU? If so, how? You aren't providing people enough information to evaluate what you are proposing. Thanks, jtoomim (moderator)]
Bitcoin Unlimited is proposing that full nodes and mining nodes decide the blocksize settings for themselves from among BIPs or their own custom configuration, with Bitcoin Unlimited being a vehicle for doing that more conveniently (users don't have to mod the code themselves).
The idea is that users would converge on a consensus Schelling point through various communication channels because of the overwhelming economic incentive to do so. The situation that results would be no different than now except that there would be no reliance on Core (or XT) to set what the options are. Bitcoin Unlimited, as software, constitutes a meta-proposal that rejects the idea that it is the job of Core (or XT) developers to govern policy or available policy options on blocksize. Bitcoin Unlimited would relegate developer-led BIPs to the status of mere recommendations.
BU is proposing that full nodes and mining nodes decide for themselves. BU is a vehicle to permit its customers to easily adjust their block size limits. It will also be a tool to efficiently communicate those limits across the network, thereby facilitating an emergent consensus.
Edit: comment no longer applies as ZB has updated.
The guiding principle for Bitcoin Unlimited is that the evolution of the network should be decided by the code people freely choose to run. Consensus is then an emergent property, objectively represented by the longest proof-of-work chain.
I mean I didn't include the part about communication within the client, but if that is a planned offering it probably would sound better with it in. Not sure whether to mention acceptance depth. Depends on whether it's seen as particularly important for arriving at Schelling consensus or as just a useful feature.
Funny comment, because the game theory of the Core dev team "consensus" process is even less understood (to put it lightly), and if we're not restricting the options to Core then the exact same game-theoretic issues arise in the choice of Core vs. XT vs. whatever (or if it's solely the choice of miners, the same issues apply among the BIPs - except without the added friction of user inconvenience). No matter how you slice it, BU makes for the cleanest game theoretic considerations.
If you don't want to use my text above, can you replace the existing text with my revisions here?