One way to explain, OS upgrades are always best experienced on AOSP based devices like the Nexus or when running a AOSP based custom ROM. That is where you get the true taste of what the OS was designed to be enjoyed as. The OEM's like Sammy, HTC, Moto, etc.. come in with their UI overlays (Touchwiz, Sense, Blur, LG's own) and by the time they slap that over the top of the new OS you really don't see much of what Google intended. It comes down to more of the under the hood tweaks/upgrades at that point that are left for the end user to experience because the OEM has changed the face of the original Google design.
Its an upgrade you want, because naturally you need to keep moving forward in time to stay in tune with the upgrade trends for future applications, etc that are built based on JB 4.1.1 but your not going to see the dramatic change that you would see if it was AOSP.
Probably the biggest visual changes we have seen in non AOSP devices has been from Eclair 2.1 to Froyo 2.2 (Froyo gave us Flash Player support) and from Froyo 2.2 to Gingerbread 2.2.3 (which brought in the famous GB green fonts & colors along with the end of screen over scroll glow effects we now take for granted.) GB to ICS wasn't a big deal and ICS to JB (Jelly Bean takes away Flash Player support) won't be much in visual aspect either.