Shrike
Warning: May Contain Nuts
A clever writer would have exploited that. For instance, you could still have a blindingly obvious solution that eluded the protagonist, and simply establish pity for the character. As you've said, many people have been in a similar situation of expert's blindness, where a simple solution is overlooked because it is too basic. The character would be seen as more sympathetic, allowing for good contact between the reader and the protagonist.
There's nothing wrong with having such things in a story such as mirror-writing codes. What is poor with DB's writing is that he doesn't seem to recognise the nature of his own writing and what it says about the narrative.
Athon
You're quite right.
I really should get down to the library and start reading them now
At least it'll give me a reference or a base line of what is bad writing.