I'm a big guy, and when I'm in shape, which may or may not be the case, I can usually work out with 40 lbs dumbells for bicep curls, and up to 60 lbs for chest...
So the weight doesn't really affect me all that much, but if you have an extra lbs or two hanging from your neck over 8+ hours, it can start to give you a cramp... and your wrists and forearms can start to ache... mine too... but I just ignore the pain.
For an hour... most people can deal... but over the course of a day... then that starts to wear on you.
As an example... when I played football, I would be in incredible shape before camp started... but wearing the helmet for two a day practices cause a pretty severe ache in my neck and shoulders.
Maybe it's just that different fram of mind then. I don't mind hurting, if it's the right tool for the job. I'm definitely not a big guy (5'10" 150lbs) and I find I hurt whether I am carrying two bodies with primes, or my 70-200 F2.8 and 24-70 F2.8. I figure If I am going to hurt either way, I may as well hurt while having the best gear readily accessible.
I suppose I could see weight being very important for those that do a lot of hiking with their gear though.
I'm a pack mule for out family, but if you and the family ever go to Disney World/Land, it is hard to take just one lens, but with cold water in the bag, souvenirs, those stupid pins, your phone, your daughter's camera that she decides she no longer want to carry, a bag of sugared nuts, etc... it all starts to get heavy. I didn't have my 70-200mm f/2.8L IS mkii the last time I went, but I might be hard pressed to have to CARRY ALL THAT plus a few lenses. So I will have to plan accordingly.
Thanks jdramirez for what you wrote - I appreciate the personal touch and 'hands on description' - that you've given, and what works for you (as roadrunner has also posted)
I'm a slightly smaller than average man (171cm, 62kg)- reasonable level of fitness / stamina, but I'm not big / muscly by any stretch of the imagination.
Over the years I've taken tens of thousands (if not hundreds of thousands) of photos - many at camps, and on outings, etc. When I started with my first DSLR (about 8 years ago, with a 350D). The 350D with 28-135mm lens which I used most of the time (or even the 18-55mm kit lens) was of course 'huge' compared to my previous Fuji P&S.
Now I own a 7D also, and that's my go-to camera for most things - along with my 15-85mm lens. I can (and often do) use the camera for many hours at a time. I recently bought my fiancee a Sony RX-100 - a great P&S camera - larger sensor than others in its class, with good optics.
As much as I enjoy photography, I certainly appreciate times when am at an event, or walking, or sight-seeing and I do NOT have 1.5 kg of camera & lens dangling from my neck (or being held in my hand). That's the main reason I got my fiancee a true pocketable P&S, because I know she'll appreciate the smaller size of that camera.
So...... back to the original topic: rumour of Sigma producing a 24-70mm f/2 for FF. I believe laws of optics determine that the lens would need to weigh substantially more than any existing current 24-70mm f/2.8 lens. Therefore a 24-70mm f/2 might not be an ideal 'light / travel companion' to eg a Canon 6D as your travel lens.
That's where I see the 24-70mm f/4 L IS (or the 24-105mm f/4 L IS) fitting in - ie for those doing a lot of travelling, and where IS is more convenient than a tripod. (And that's the same reason why I on occasion even use my 18-55mm on my 350D as my 'long bushwalk / travel combination' - where weight is really a serious consideration, even though the grip on the 350D isn't as comfortable as that on a 7D).
Again, back to a rumoured 24-70mm f/2.... For those interested in the ability to control DOF and/or shooting in low light- a 24-70mm f/2 would have an envious amount of flexibility, particularly if it was available with OS/IS. And if it has superb high quality optics - that would be the dream lens for many on a FF. Roll on Sigma!