The most critical and important aspect to handloading to you?

I think without a doubt it is bullet seating depth.

How do you find your proper depth without a "depth gauge" cut with the same reamer that chambered your barrel?

Myself being a new reloader there is no single most important aspect. I am cautious through every aspect of the reloading process. With that being said I take a bullet and seat it in a primerless case. The bullet depth is adjusted so when I chamber the round it will push the bullet into the case. I very slowly remove the cartridge from the rifle. This will give me a good starting point for that particular bullet as it inidicates where the lands start for that particular case and bullet combination.

I use a Hornady OAL gauge and bullet comparator to find the lands then measure with digital caliper. I log this into my Speer Manual for every weapon that I load for.

Bullets! Sorting them by Ogive to Base length within .001�. Meplat uniformity either trimming or trimming and pointing. Final weight sorting them within .01 of a grain.
Seating depth is very important also. I seat a bullet long and chamber it, leaving distinct land marks on it. I do this each time I set up to load, checking to see if the throat has eroded. I set my seater die up to set the bullets .008� jam in the 6mm Dashers and .015� jam in the .300 ATLASs.