Hi Maxian,
here's some feedback
It's funny that I was also a heavy MazzleUI user - even being a beta tester almost from the very beginning, however, since Mazzlefizz unfortunately left the game some time ago, MazzleUI has had a number of problems especially after patch days, I've decided to change UIs after a lot of thought (even though I still like it). I now have a much simpler UI, which I must admit does have some limitations in the way that I play.
I like the general clean, sharp look of the UI. The tooltip looks great and is informative, the damage/healing text on the right is informative yet compact at the same time which is great. Less used items are at the top of the page, which is a nice change. Overall a very nice looking UI. However, there are some things which I would personally rethink. Just note that these are my personal opinions, so yours or anyone else's might differ.
Anyway, my current UI's minimap is at the top left-hand corner. I can't get used to this and I find that I often miss out on mining nodes or herbs, since on my resolution (1680x1050 just like yours), you've actually got to move your eyes around to see the minimap - you can't see it from the corner of your eyes. In your UI, I might be tempted to move the minimap closer to the middle.
Unlike yourself, I'm not much of a raiding person, so the space which is usually occupied by the grid, is kind of wasted in my case and perhaps that of others. Perhaps this might be made more useful for the less raid-orientated people. I might also scale down the grid to make it align with the rest of the bottom part of the screen.
I'm a fan of symmetry so having the target unit frame not aligned with the player unit frame sort of stands out in my eye. I would align the player and target UFs with each other and place the ToT UF above them somewhere.
Other than that the UI looks really nice. I might be tempted adopt it myself
Good job!
Oh, could you possibly post a screenshot with just a normal party?