Three things: fit, fit and fit. Okay, two more things: a padded back panel (unless you can sleep on a plain foam pad, in which case you are a tougher person than I) and a very good compression system that takes up unused pack space as your load empties.

I've used two comfortable-for-me frameless backpacks: GoLite Jam and Granite Gear Crown 60. Both have padded backpanels, sophisticated compression systems and fit me very well. Because I like their light frame packs, I'm guessing ULA frameless are likewise comfy (and they have myriad fit options).

I'm sure there are others, but I've not tried them. I have had miserable hikes with frameless packs that don't fit well, so I'm pretty leery. Oh yeah, the Osprey Hornet 44 has a minimal rod frame at a frameless weight.

Cheers,