Anyone care to speculate on how you'd make the zombies a bigger threat without converting them into fast zombies?
Because everyone in the group has developed skills on how to kill them - even up close - without fear or even a gun
The S2 finale is an example of how the zombies can continue to be a bigger threat. In that situation, you had a number of armed people unloading on the walkers:
But their efforts ultimately meant nothing as they were overwhelmed.
In the pilot, and the last two episodes of S1, we saw many tanks and soldiers out of commission / dead, respectively (the city and outside of the CDC). I'm guessing large herds overwhelmed groups--even armed with military firepower, so if that guess is spot on, there are still ways for zombies to pose a great threat.
Even at the prison, the survivors were always ducking and dodging in the commons area, the halls and in the yard, as the numbers and walker hunger keep them chasing the living.