In my opinion, one of the most important and most difficult parts of the job. Architecture and design shouldn't be limited to senior engineers--it won't be in practice, anyway. Doing so is a sure fire way to stilt the growth of your team.
But, reviewing designs is hard. It requires recapturing much of the context that the engineer gathered in a very short period of time. I also find it sometimes difficult to separate, "this is a fatal design flaw" from "this isn't how I would do it." I really like the suggestion of providing feedback via additional information.
Mistakes are a very important part of learning. I try to make sure everyone has the opportunity to make their own instead of making mine.