I don't have as much experience with camera bags, but I did own the Toploader 70AW (Street & Field) for over 15 years until last summer when it was stolen (along with a certain wide angle zoom lens) last year while vacationing in Italy.
As many of you have said, what I liked about the 70AW was its solid build. The one thing I didn't see mentioned were the metal latches.
I decided to replace my bag with the new Toploader Pro 75AW (knowing I was planning to add a 70-200 lens to my collection). At the time it was priced at $110Cdn, which was about the same price as the older series. I think that price has skyrocketed to about $170Cdn now.
When I brought it home, my initial thoughts were:
1. Cheaper latches. (Hard plastic vs. metal)
2. Only one lens attachment.
3. Weaker "grab-and-go" latch (the short handle). This one "lesser" feature alone almost incented me to return it for the older style.
Having put the bag through the paces, I will tell you what I really like about the bag:
1. I really like the ability to close the top with a quick snap (as well as have the option to zip afterwards). Most of the time, I only close the clasp if the camera is going to be in and out several times.
2. The side opening. With my old one, I carried it with the chest harness (which was a feat to get in and out of). When I opened the top, it covered my face instead of opening outwards (permitting easier access). With the Pro version, I carry the bag messenger style and the side opening is perfect. However, I could see this being a slight problem if you carry the bag on your right side hip.
3. The extra strap the cinches around your waist making it comfortable to carry it messenger style.
4. Although I miss the ability to carry an extra lens attachment, I have found the web pocket to be very useful for tucking away lens caps in a jiffy (no more lost ones, or searching for a vacant pant pocket) and have also used it to place my Epson viewer, or my smaller P&S (for video).
5. I like to use the additional top storage pouch for my TC1.4 (There's a slight bulge, but not too bad. I wouldn't want to throw anything taller like a 1.7 or 2.0) as well as a portable Targus cable lock.
I have only had the new bag for about 9 months, but other than the cheaper short handle, I have never felt that the new style bag was not sturdy enough for any of my lenses. I have had my 2 year old daughter often sit on top of the bag while I hold her on my hip when we are walking about on vacation and do not feel apprehensive about any of the buckles breaking.
One more thing I will mention. On the front pocket, there is a small opening which (I presume) was designed to tuck away a LensPen cleaner. While neat, I have already lost one of them this way (too easy to unclip, unless my kids grabbed it without my knowing).