"Validation" is not an issue on its own. This is just a subset of eventual consistency issue. Generally speaking, CQRS is suffering from this, not necessarily the event-sourcing.
I disagree and don't think this is a problem specific to those patterns. I think it is a human nature problem of getting a new data hammer and then every bit of data looks like a nail. It's part of how we learn appropriate uses of the tools, but it's nice when somebody can save you the trouble of learning the hard way.
This is not what I meant. Essentially, the "validation" issue comes each time you have any bit of eventual consistency, where you have to check the system state in a potentially stale store. CQRS is more often eventually consistent and the question of validation comes as a winner in the DDD-CQRS-ES mailing list and on StackOverflow. However, event-sourced system can have a fully-consistent index on something that needs to be validated and therefore this problem can be removed.
I understand what you are saying, but I didn't want to conflate the issue with the universe of DDD/CQRS things. I just wanted to point out that sets are one of the things that log based storage does not do well. Go ahead and look at relational tables for that. Either as the source of truth for set data or -- and I like how you put it -- as an index from log storage.
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