Jump to about the 19 second mark of this video.
You will notice Ericsson "covering" Joe Thornton.
Now don't get me wrong, this is "Jumbo" Joe Thornton. He is one of the best playmakers in the league.
But there's simply no reason to back off of Thornton like Ericsson does here.
Smith is usually in the right position either in the offensive zone, or defensively, and this is evidenced by the "zero" rating his has in his plus-minus rating through four NHL games.
Ericsson by contrast was a minus-19 over his first 89 NHL games.
Positioning is one of the single greatest things that a defenseman can have over an opponent, and watching the greats like Nick Lidstrom this point is emphasized even more.
Being in the right place at the right time makes all the difference, and at least for the moment, Brendan Smith seems to have this aspect on Ericsson.
If positioning is the best attribute that a defender can be known for, the second most important attribute is on the next slide.