I'm getting ready to finish the basic kitting of my bike tomorrow. I've already got a computer, water bottle and cage, and rear light: tomorrow I'm getting the front light, mirror, rear rack, and a stem to make the bike fit properly...and possibly some different tires.

The essentials, for me, are rear light, front light, and mirror, and making the bike fit properly. The rest can be worked around.
I say a mirror is essential gear because ears won't tell you how much room the car's going to give (or not give) you when they pass and hybrids are both really quiet and getting more common.