We've had atleast two free agent receivers coming off good (by their own standards) years and then completely fail here -- so it's likely that all rookies get strenuously tested for the mental aspects, so Bill believes the draft picks he selects can learn the Pats system -- but maybe he doesn't test free agents or trades in the same manner -- as there often is a rush to complete the signing.
It's quite possible that Browner isn't learning the Pats system instinctively -- I don't think Sherman was saying the Seattle system is 'do whatever you want' -- or even simple -- but that it's not based on a lot of individual reads and assuming everyone on the defense all makes the same reads. So it might be 'this play is going to be a cover 2 man; or this play is a cover 6' and then the players know what that is and they can execute it without thinking.
If the play is 'if the receiver lines up here, then do this; if the receiver runs a slant, then play it this way' then unless you're able to learn to process all that information instantaneously, you'll be thinking instead of doing during the play.
I think one glaring fault of BB's system is to not bend it to the strengths of his players. It took them 4 weeks to really figure out what was obvious after seeing Revis play with the Jets and with Tampa.
And for offense, it would seem to me that its even easier to bend the system. If a receiver is able to be productive, but just can't learn to adjust their pattern based on the defense, then you get them a set pattern and let the other receivers react.
On D, maybe reducing 'during play' decision making gets more overall productivity