Obama administration has been ignoring North Korea since taking office. The artillery shelling and 2000 centrifuges in operation to enrich uranium by North Korea merely wanted attention from the US, South Korea and the West to get them to return to the 6th party talk with more bargaining chips.
Although North Korea has the numbers advantage in military might but South Korea has the superiority in military advancement weaponary which is an edge over the North. However, half of South Korea GDP are located in and around Seoul which is about 50 km away from the DMZ border. Therefore, South Korea cannot afford a full force confrontation with the North.
Besides, North Korea has nothing to lose (One third of the population is in the Army and the rest are mostly poor peasants).