What exactly is the danger for Obama in chasing the "liberal Reagan" mantle?

Yesterday, I offered a partial dissent from Jonathan Bernstein's contention that it's a myth that Ronald Reagan "defined an era" or, to borrow Obama's 2008 phrase, "changed the trajectory of America."  Reagan, I argued, did set the ideological mold that subsequently hardened into GOP dogma, not least by at least appearing to demonstrate that tax cuts can unleash economic growth.

I'd like now to probe a little deeper into exactly what Jon was warning Obama against. I'm not certain, but I suspect that the warning may pertain to a perceived flaw in Obama's current political strategy fingered by Ezra Klein and others. Eliding out the meat of Bernstein's debunking of the "transformative Reagan" myth, here's his advice for Obama:

Here’s the problem. Ronald Reagan wasn’t really the Reagan of everyone’s imagination. So aspiring to be a “liberal Reagan” is chasing a fantasy. Worse than that—it’s a fantasy that can easily distract a president from the real things that he should be doing....