What are some reasons that a rational miner won't switch to P2Pool? Besides the initial time investment of downloading &amp; learning how to use a new tool ... what are some more reasons for a rational not to switch to P2Pool?

(Some reasons to switch is a slightly higher payoff in P2Pool due to bonus donations, and the overall contribution to a more distributed Bitcoin network).

Is running P2Pool more maintenance than using one of the larger pools? Is it more buggy, or has a lower total hashrate and thus higher variance?
Variance. Raw connection to p2pool will always have variance for a typical miner - if the pool is small there will be large pool-based variance, if it is large the share difficulty will be high and there will be large share-based variance.
Running a Bitcoin node is already nontrivial and going forward will become impossible for at-home miners. For example, some mining rigs run on a cheap USB flash drive, which may soon become insufficient to hold the blockchain.
Normal mining pools have additional features such as worker monitoring, SMS notifications, automatic conversion of NMC to BTC and so on.
Some of p2pool's suggested advantages are a bit exaggerated. The recent P2SH woes are a non-issue since miners who want to vote can simply go to a pool that votes as they want. The more generic concentration of power problem can alternatively be solved with smart miners (miners who generate blocks themselves or from an independent party, and submit shares with Merkle branches of the generation transaction which are accepted if it credits the pool).


Because of these reasons, I believe the future will be small PPS pools which act as a proxy to p2pool (with or without the use of independent block-issuing nodes). These will have low fees, no variance, as many features as the pool operator wants to implement, and will not be highly centralized. Until we see more of these I don't think there's much justification for p2pool going mainstream.