The last thing a DE should do is consume so many resources nothing else can run properly. There is no benefit to it. DE's should use almost nothing and run lag-free on 10 year old hardware. Show me a single feature in Unity that saved so many people, so much work that it justified extra power consumption across the world. Hell, show me a single feature that didn't actually slow people down compared to, let's say, xfce.

1. Global Menus — by being at the edge of the screen, their "height" effectively became infinite (for the purposes of Fitts's law), so you could very easily and quickly move your mouse to them. However, if they annoyed you, you could turn on Locally-Integrated Menus (at least in later versions of Unity).

2. HUD (Heads Up Display) — by just tapping Alt and typing, you could quickly select an option from almost any application's menu (saving you the time of looking through the menus and trying to remember whether the desired option was under Edit or View).

3. Not a speed benefit, but: better use of vertical space.

BTW I think that XFCE is a great DE.

Those are nice, though #1 breaks down with multiple monitors. None really required a new DE, could have been added to any of the existing.

>I think a lot of it just came down to performance.

Absolutely. I for one embrace (positive) change and had no problem adjusting to Unity. I liked it a lot, even more than Gnome 2, but the performance was just shockingly bad on every computer I used it on. From my Thinkpad T420s with the Intel graphics to my desktop with a GTX 970. Everything was noticeably slower than it was under any other desktop environment. I finally just quit using it. The real shame is they had a version based on Qt a while back that didn't require compositing. If they had continued with that, the performance would have been stellar. Oh well.