The new design is based on web browsers, so it is more familiar to people. I think in general it is a great new interface. More simple, easier to use. However, there are quite a lot stupidities that made it very inefficient while it need not be. Here's some examples i'm writing down as i find it. (i haven't been using Viewer 2. I always use Emerald and now Phoenix.)
Keyboard Shortcut Problems
In old viewer, you can press 【Ctrl+i】 to always expect the inventory to come up. Now, not so. When you do 【Ctrl+i】 now, it simply brings up the sidebar of whatever info was there last time, such as properties of a object. You now have to use mouse to get rid of that. 【Ctrl+w】 won't work.
In old viewer, press 【Ctrl+i】 and your cursor is in the search box so can type what you want to find things right away. Now, you have to press 【⇧ Shift+Tab ↹】 twice to put focus on the search field.
If you pop out the inventory from sidebar into a standalone window, then, the 【Ctrl+i】 shortcut no longer works. Nor does 【Ctrl+w】 work.
The 【Ctrl+t】 to get last IM chat is gone. There does not seem to be any keyboard shortcut to bring up the IM chat window. So, when you have multiple IMs, which happens often, you have mouse click each one, a pain in the ass.
You can't see the region coordinate of your current position anymore. (you need to turn it on, under menu 〖World ▸ Show ▸ Coordinates〗)
The group notices are better, but… When you have a notification from group, it shows in the corner 2 lines of it, and when you click it to read it in full, it shows in a tiny window. The Windows cannot be enlarged. You have to scroll. And, before you got a chance to scroll, it automatically disappears due to a few second timeout. What a idiocy.
The pie menu has been replaced with the usual rectangular menu. Pie menu is much easier to access. (See: Pie menu)
In old viewer, i can see all people on my friend list who i have given map or and edit rights. I can easily edit them. With Viewer 2, you have to do a few mouse clicks to see that info for one person only (Click on info icon, then click on Profile, then “NOTES ＆ PRIVACY”).
The ability to adjust each person's voice level seems gone.
Mouse Look Problems
When using show beacons 【Ctrl+Alt+⇧ Shift+n】, but when in mouse look, all disappears. Quite stupid. When fixing griefer problems, or in combat, having beacons on while in mouse look is very useful.
When you are in mouse view, you can no longer see your altitude coordinate. So for example, when you are flying or using jetpack in combat, you can read off your altitude. But now, when in mouse view, all GUI are hidden.
In mouse look, you cannot look all the way up at above your head (zenith), nor all the way down. Looking at the zenith is useful when you need to throw grenade above a wall almost vertically above you.
Other Bad Aspects
Here's some issues with Viewer 2 or before that i hate very much. Been using
Emerald Viewer for 2 years, and now Phoenix.
Teleport blacks out the screen. VERY ANNOYING.
Very annoying teleport sound.
No sim-wide radar. In Phoenix, you can press 【Ctrl+⇧ Shift+a】 to list all people in a sim. In Viewer2, it list only nearby people.
What are some UI improvements?
Overall, the GUI is more intuitive and modern.
The notices are better. You don't have the old one stack on another.
The lsl menu are better (menu from your attachments). It is now shown as a standard window. You can close it in normal ways such as clicking the X on top right or use 【Ctrl+w】. (But you need to click on it first to put it on focus.)