It seems to be a UI that favors clicking, am I correct? I don't see any keybindings, but it could be that you just hid them.
Either way, I'll judge based on my assumption being correct, in which case your action buttons are really small and waaay out of place. You shouldn't have to move your mouse that far to then have to click really accurately or you'll use the wrong ability, that's a design flaw.
Also, healers will struggle a lot with this UI, because raid frames suffer from the same issue. The vital information a UI provides should be as close to the area where you spend the most time looking, in 99% of cases this is the middle of the screen where your character is, because positioning is so vital in this game. Having raid frames all the way to the left of the screen is a huge problem, especially on a widescreen monitor where you'll have to move your eyes/cursor even further to get the information you need.
If it's not made for clicking, why show so many buttons in the first place? It seems like a waste of screen estate to show such an enormous amount of buttons, especially when several are of the same spell (Intervene appears 4 times in the same general area, but not all next to eachother for easy clicking either).
You should provide pictures from a casters point of view aswell, to show where you've put castbars and such. Don't forget to include target castbars too.
You should consider organizing your WeakAuras or whatever it is in the middle of your screen. That Retaliation icon is going to overlap nameplates a lot, as it does in your screenshot.
Finally, if I'm nitpicking, you should also consider changing the timestamps in your chat to not include seconds. You don't have to know which second a message was sent, that's incredibly irrelevant unless you partake in... I don't know, auctions via guildchat?
