There's been a few recent posts on the Lucas critique in the economics blogosphere (ex: Noahpinion), so I'm going to take advantage of this to make a point that I don't recall seeing made elsewhere. (If I'm re-inventing the wheel here, please let me know in the comments and I'll add the appopriate attribution.)
A key insight of behavioural economics is that people don't always and everywhere re-optimise whenever their environments change. Instead, they will often - or even usually - make use of various rules of thumb and/or passively accept the default option. The costs of re-optimising every time you face something new don't always offset the benefits from making what may be only a slightly better choice. This the idea behind 'nudges': you can alter people's behaviour by making minor changes to the frames in which people operate: if people have the habit of choosing the default option, then you can change choices by changing the default option.
But this only works if the change is subtle enough to not attact the full, direct attention of the decision-maker. If the change is big enough, people will haul out the full artillery of their rational selves in order to try figure out what optimal decision is. This means that behavioural economics is unlikely to be of much use in policy-making.
This is essentially the point of the Lucas Critique. Behaviourial economics' prescription of nudges work precisely because peoples' attention are not called to them, or how these measures are supposed to work. But if you announce a measure to change people's environments in order to attain a stated policy goal, peoples' attention will (presumably) focus on the problem at hand and they will re-optimise. This reaction - the focus of attention and recalibration in the new environment - is exactly what nudge prescriptions are designed to avoid.
Okay, you may say, that may be a problem for people like you and me, but most people won't be paying attention to the policy discussion in the first place, so the nudge story still goes through. But I'm not sure how much I'm willing to get behind a policy whose effectiveness depends on citizens' not knowing what their government is doing or why.