1. AP doesn't have the facilities for global aggregation which can power search, discovery, algorithms, and metrics. The user community has been very clear that they do not want it to be introduced. We felt the connectivity of a shared global network was extremely important to the UX, but we felt it would be wrong to fly in the face of the AP world's established norms & wishes.
2. We felt that strong account portability was an extremely important feature of the system, to ensure that users don't get locked into a specific host. AP's redirection model of account migration concerned us.
3. We're concerned about the cost structure of AP. We're concerned that self-hosters are going to pay a prohibitively high price for virality. This is why we designed the network to avoid placing heavy load on PDS.
I know that the AP world is frustrated with the competition between the protocols and suspicious of how we've chosen to do things. It's a shame because I think we're after similar things, and hold similar values. We didn't set out to sabotage the AP world; we just felt like there were important changes that needed to happen for this mission to work.
Note, however: Our software is not proprietary. It's open-source. The specs are open. The network firehose is open. We're working on getting every piece of the infrastructure into good governance and straightforward self-hosting. It just takes time.