Well, sometimes hobbyist photographers are the most-discerning photography practitioners. Sometimes it really,really helps to have the exact right lens for the job; if you need a 300mm lens, 300 is great; if one shoots baseball of field sports and needs a long reach zoom, then an 80-400 or 100 to 400 stabilized lens is the right tool for the job.
As far as differences--the older Canon 7x-300 IS lens was clearly not a very good lens, and even on casual shots, the purple fringing it showed on my buddy Steve's early Rebel days shots was pretty obvious. Same with Nikon's 70-300 G series, the original, low cost one, not the new VR-G model--loads of purple CA in that lens, toward the longer end of the zoom range. Both Canon and Nikon have really stepped up their game on these new high-end consumer 70-300 lenses with stabilization; these lenses are very capable,new designs,designed for today and the future.
IQ differences among lenses are largely masked by 6, and 8 and 10 MP sensors in my experience; on APS-C, 12 megapixels and up starts to show the better lenses as being better lenses, and the lesser lenses show that they are lesser lenses; I would say buy with the future in mind when moving into these $800-$1000 class zooms. Right now it's 15-18 MP in Canon APS-C at the medium price point.