Just a couple initial things I noticed that felt awkward and an “okay, not as bad as I thought it’d be”. 
All my settings from the previous Roku Preview version transferred just fine to this version. That was nice.
When it first loaded defaulted to HOME it said the admin hadn’t loaded any libraries and even leaving and coming back in or navigating to settings or along the top bar changed anything. It wasn’t until I navigated into one of my libraries the first time that Home then populated. That was a little odd. I have all discovery stuff disabled so only had Search, Home and Libraries so maybe that was part of it.
Two similar navigation hiccups:
1st: When I go to my libraries and I want to “heart” them. I put a heart on the first library and then I click “down” on the remote to click the heart on the next library… but it doesn’t take me to the heart it takes me to the library name to navigate to it. Same thing happens if I’m on the hearts and click “up”.
2nd: Similarly, if I click on my profile picture and then click “down” it doesn’t take me to the profile icon below my profile picture, it takes me to my “details” box on the far right instead. I have to click down and then left to get to the left side navigation for the profile stuff.
In both cases It isn’t going up or down in the column I’m in, it’s going to a different column instead - if that makes sense? The focus moved vertically and horizontally when I expected it to move only vertically. It’s not intuitive and it made it awkward.
It’d also be nice to re-arrange the favorited libraries - or maybe put a note in some of that expanse of blank screen space saying something that the order they are favorited will set the order of things on the Home Screen. It is particularly irritating redoing favorites to re-order them when it doesn’t go up and down the hearts column directly. 
That top navigation:
I do not like the top navigation - my TV is wide, not tall, and it feels like wasted screen use taking up a dedicated top bar pushing some info down when there’s blank space on the left hand side of my screen under my profile picture for those buttons using those icons.
Speaking of the top navigation… here’s the "not as bad part … hitting back on the remote (the dedicated button next to home, not the left arrow on D-Pad) does jump me to the navigation no matter where I am - including leaving the libraries open while I navigate - and hitting back again jumps to home. That is working better than what I saw with the AppleTV Experience so I’m glad it’s working this way here. It still feels weird to have a left facing arrow on the remote go to a top menu instead of a side menu but… shrug.
A back button oddity that felt awkward as well. If you hit BACK enough times Plex opens a pop-up asking you if you want to exit or switch user. You can hit back again to exit that screen but that doesn’t feel intuitive. If you give 2 options to select on an overlay it might confuse people who won’t think to select neither of them and back out by hitting the button that got them there again. Maybe move the switch user option to somewhere else and let that be a “don’t exit” or “stay in Plex” or something… or add that as a 3rd option. I’m not really sure where that works best but I think something a little more clear would be helpful.
Lastly… for now:
I’m with @cmpegeek that finding some happy medium on the preview image layout would be appreciated. In the current Plex I’ve learned to pick my backgrounds knowing the right side will be cut off in the preview display on Roku so now cutting off the other way feels weird and messes with my chosen backgrounds a bit now. I assume some of this adjustment maybe is to address people with background art with a focus on the right side - common so it works well with text summary\titles on the left (which also seems common default options too) - and complaints about art getting cut off so from that assumption the change makes sense… just… would like it to not cut anything off if possible?
Edit: Sorry… one more pro column thing; was showing the new layout to the wife and it is definitely snappier. The transitions are clean too. My RokuTV is not very powerful either and it’s is noticeably snappier moving through the interfaces.
That blank left hand section on the screen just looks odd still though… just a profile icon? The profile icon disappeared after Roku went to sleep so it looked even more weird.