Stuff like vertical grips, railed forearms, flat top receivers, bipods, various flash suppressors, bull barrells, fluted barrells, flip up sights
Just my personal preference...YMMV of course.
I agree that the vanilla AR15, such as the one you pointed out, is a beautiful weapon. I'd happily take them in pairs.
I'm kinda curious about your preference. Is it based on the cosmetic appeal or the functionality of the items? I think the flat top receiver, flash suppressor, and fluted barrel are all far from nonsense. I also think that railed forearms are a great choice.
Please don't think that I'm accussing anyone of being wrong or right. We all have our own individual tastes and preferences. I'm just curious.