I also had a modern plastic 18-55 mm kit lens, high f (starting at 3.5).
But the colors on the modern zoom lens were so much better. Hard to explain, but they just looked better, kind of like a DSLR photo has "better" colors than a phone photo.
It was very confusing, I was expecting differences in sharpness maybe, but not in colors.
It really is incredible how much a lens affects the final image.
I had a Canon 40D a while ago, and at first I was using a 17-40/4. I absolutely loved the colors I would get. Gradients would have a certain "creaminess" to them.
Then I figured I'd change it for a 17-55/2.8, to get some extra light at night. That was pretty much when I stopped going out taking pictures altogether. I just wasn't getting the pictures I wanted anymore. Everything looked just... weird and unsatisfactory. I then sold everything a when I realized I hadn't touched the camera in a year.
A few years later I got to do a similar comparison again, between the Olympus 17/1.8 and the Panasonic 20/1.7 on a Pen-F. The panny was OK, sharp, etc. But there was just something missing that the oly had. A kind of delicate rendering of gradients that just appealed to me.
This was when I realized what had happened with the Canon change earlier. I had believed that I had just lost interest in photography or that for some reason I just "didn't have it" anymore. I had never thought that a lens that was rated better in practically every aspect would actually be so uninspiring to me.
Even just fixing the vignette some lenses add is enough to make pictures no longer depressing. If the lens adds chromatic aberration that's quite difficult to fix though.
I actually tend to add vignetting in post-processing, so that's likely not the cause. The issues I had were with color rendition, and in particular with subtle variations in tone.
I don't remember there being any strong chromatic aberration either. I remember that both the 17-55/2.8 and the 17-40/4 where quite highly rated at the time, with the former having a better measured quality than the latter, even on a crop sensor.
Don't get me wrong, I don't remember there being any "technical" issues with the 17-55. It was plenty sharp and aberrations were kept to a minimum, especially since I never pushed my luck with strong backlight, etc.
But maybe that's the thing, images were maybe too "clinical". I wouldn't say the pictures were "depressing", they were just... "meh". Like random snapshots. Sharp, well-exposed snapshots, but snapshots still.
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Edit: Regarding editing, I should add that I experimented with DxO optics, which had profiles for both lenses. The corrections were quite amazing, especially at the wide end. But given my usual subject matter, I never considered the change meaningful, so it didn't bother with it in my regular workflow.