Ok, I guess the best way to test this editor is to go through the entire design process of making one level (which is attached, btw, Medieval tribe).
I'll try not to go into things that are obviously still provisional, as they are subject to change anyway.
For most part, the controls felt simple and very intuitive, making the editor very pleasant to use. The way you implemented area selection is very powerful for quickly selecting whatever you want.
I also liked the easy way to change the viewport by just dragging with the right mouse button.
Initially I had the same problem as ClamSpammer, namely pressing UDLR when I wanted to move a piece, and thought that having these keys for moving a selected piece or area in addition to dragging would be essential for me. I ended up covering these keys with a piece of paper, and got used to the mouse driven interface there; due to the 16x8 grid the mouse still gives sufficient precision easily, having UDLR for that functionality might still be a nice addition though.
The picker interface has the slight disadvantage that you don't see all pieces at once, and have to use the mouse to scroll, but I'm sure that once one gets used to the tilesets this will be less of an issue. I found a few oddities with it though:
You cannot scroll or directly select another tile/object when a tile/object is currently selected in the picker; and for placing multiple tiles/objects that do not connect you'll always have to re-select them instead of the piece staying selected.
On a different note, did you manage to find out about the precise positioning of trigger areas? I think in reveal mode these could be very useful.
I really liked ClamSpammer's level, requiring to set up some sort of roper web to get up. Due to making a few mistakes every now and then, I ended up having exactly no ropers left when I solved the level.