I just wanted to bump this with a link to the migration discussion as I think the Aave V1 to V2 migration is a very relevant example of how the aTokens are going to create a huge tax headache for existing deposits, highlighting the importance of this feature for the future.
Many people who want to (or are forced to) make the migration are going to be subject to immediate massive taxable events due to the aToken to aToken trade, especially with the huge increase in collateral value due to price appreciation, and the fact that everyone who used Aave v1 is still in the short-term capital gains period.