Damn near everything is customizable. Icons, icon sizes, animations, transitions, folders, colors, app drawer, etc. Damn near everything about your home screen experience can be made the way you want it to be.
The stock/Google launcher is not bad, but there's barely any customization on offer. It's biggest advantage is Google Now to the left of the home screens. It's also very fast and smooth, but that can also be said of most third party launchers like Nova.
If you get Nova, I'd suggest paying for the "Pro" version--Nova Prime. It offers even more options and tweaks than the free version.