NPCs are deliberately weaker than equal level PCs, so if you said "These guys are 3rd level and this is a 3rd level encounter, so it should be easy for them" things would be a lot harder than you expected. An NPC is roughyl equal to a monster of the same level, whereas a PC is built to almost always beat an NPC or monster of equal level.
NPCs only getting one at-will power is a bit of a sticking point, as it means they often have only one option to take, which is boring. And often innefective. There are good reasons why all PCs get at least two at-wills: that way they always have options.
NPCs would need extra Healing Surges, too.
And if you gave a low level NPC more healing surges and two at-wills, they start to look a lot like a low level PC. Which raises the question of what you're getting out of mucking around with the rules.