My sweeping generalizations:
I wouldn't want to haul a lightweight pack at more than 75% of it's rated weight capacity on a regular basis.
Frameless packs need careful loading techniques and lighter weights. Framed packs are a easier for a newbie Ultralighter to live with.
Getting your base weight and volume down is the key. Get a couple books on UL or study the massive information available here. Once you get the mindset, it is a continual process of improvement and tweaking
Here's my take on basic UL principles:
Take only what you will use for the trip, excluding the obvious safety/survival essentials. Leave the toys at home! Simply decanting items like soap, sunscreen and insect repellent to smaller containers can save nearly a pound.
Weigh EVERYTHING, write it down and add it up. A spreadsheet is the best tool. Do buy a scale.
Seek the lightest high performance versions that you can afford.
Seek items that have multiple uses.
Fear equals weight, particularly with clothing. Know how you body works and get the clothing layering concepts down tight. Knowing the physics and mechanics of staying warm, dry, fed and rested will reduce the fear and increase the comfort and enjoyment. Likewise for big knives, axes, big first aid kits, too many dishes and pots, etc. Do take the Ten Essentials, but be frugal and find the lighter alternatives.
Leave "fashion" and hyper-cleanliness at home. You don't need extra sleep clothing, sleeping bag liners and multiple spare socks and underwear. It's okay to be a little dirty and smelly. You can certainly keep yourself clean without taking a daily shower, and the right clothing can be washed and dried en route.
UL techniques do snowball. An UL kit allows lighter shoes and a lighter pack and you can cover more ground in less time with less pain and effort. Most of us have gone through a slow evolution of gear-gathering and many find that the very lightest can be uncomfortable and allow heavier options items like sleeping pads or fully enclosed bug-proof tents. I've said that you know you've gone too far by the TOO factor: if you are too cold, too wet, too hungry, or too tired, you may be TOO light. Hiking is supposed to be RECREATION and not a masochistic march in the wilderness.
I am of a mind that any UL kit can survive ONE heavy item, buy applying that loosely will find you with a heavy load again. I am a proponent of buying your pack last. With the narrower band of weight tolerance in a truly UL pack, it is just uncomfortable to use one at full capacity all the time and I think it puts newbies off the UL practice in general. For that matter, an UL kit in a pack built for heavy loads feels like nothing.
Have fun!