I find this a completely funny conversation. So many people on this board (probably not those already posting) claim that LGA\JFK and EWR are not the same market (ie the same passenger does not view them as interchangable). If that is the case then this move by DL is not attacking UA but rather attacking B6 & AA.
I live in central NJ and frankly whatever DL does at LGA has no impact on me on the decision I make in flight decisions. I will fly UA from EWR.
The only impact that it could have is on pricing in the NYC to XYZ market, which I would welcome, but I hold out little hope.
I look at common markets like EWR-MSP and EWR-ATL that both DL and UA compete on today at EWR. Those are very expensive flights and they have competition today.