The current problem with extensions is they break with each Gnome release. This forces a lot of user work with each upgrade wasting a lot of time (eg having to wait for author to update, picking a different extension for the same functionality, doing without).
It would also be beneficial if there were an official set of well maintained extensions - ones that if you use them you know you will be fine through upgrades etc. This also solves the problem of there being so many extensions (you get 3 pages of search results for system monitor - how is any user reasonably expected to deal with that?)
The area where Gnome shell is seriously lacking is for people who have multiple monitors, and/or have many windows of the same app open. So far a panel/taskbar has turned out to be the least worst UI way of handling this. That is why I use fallback mode.