I'll be honest, I don't see any great difference between GIMP icons and Inkscape's. GIMP icons seem to be an attempt at 3D, but they're so small, the 3D effect is hardly noticable. It's really hard for me to see that much difference.
I'm about to make some comments that might seem to be judgemental. However, before I do, please understand that they're sincere, and not intended to be judgemental. I just don't understand how the appearance of icons in a graphics programs can make or break a program. Shouldn't the program's functionality be the main focus?
I guess my preference for icons would be that they indicate the function of the tool or button, quickly at a glance. And that is something that I think would be hard to design, given human individuality. I suspect that no cache of icons will satisfy all users. But if I'm going to be using a program quite a lot, after a while, I would learn that this icon means this tool, despite what the icon looks like. For example, the Tweak tool icon. I think it looks like a wave on a gray ocean -- not quite a good indicator of what the tool does (although I do get the similarity). But I do know that "gray wave" means Tweak tool.
Indeed, having recently healed from cataract surgery on both eyes, I'm just now really able to determine what many icons look like (not just Inkscape, but all kinds of programs and websites). It's actually been kind of amusing, when I realize what an icon is and what I thought it was, lol

Another example, the 1st six buttons on the Node tool control bar. They just looked like a series of dots and dashes, and I relied heavily on the tool tip popups at first. Now I can see the tiny arrows showing the "before and after". But the bottom line is that I always knew the function behind the icon, just by repetition. Aesthetic value of tool and buttons just doesn't seem so important to me. But, continuing with my honesty, perhaps this shows my age. Perhaps a younger me would want more "
coolness"?
Anyway, I'm curious, like prkos, what is it about the icons that you don't like? Maybe I have misunderstood you?
PS - Rob, where are these?
the funny hand, grab pointers
Edit -- All that said, there have been a few other negative comments about Inkscape's icons (which as I said, I don't get) over the years. Maybe someone could develop several icon packages (or several people could develop icon packages) for many different personal preferences. Sort of like "skins" maybe. So those that are somehow offended by default icons, can have a choice? Maybe a nice little project for those with coding/development skills who don't have enough time to commit to overall development??? (And maybe those artists among us could create the icons themselves.) Maybe you'd like to submit a wish list/new feature request "bug" report? Or I will if you don't -- if such does not already exist.