A great entry by Liz Garnett over at Helping You Harmonise about the question "How did the performance go?" She really hits the nail on the head, in my opinion. Performing as a conductor requires a different state of mind than rehearsing, and being deep within the music makes it difficult to get a bird's-eye view of the whole thing. This plays into what I call the "input/output" problem; I have problems doing both at the same time, so often in rehearsal I might just conduct passively and analyze the sound, whereas if I'm performing-conducting, and really sending out my energy and intentions, it's much harder to analyze the input in the same way.
Hat-tip to Allen Simon at ChoralNet's ChoralBlog.