I've been using Xenko for almost a week and I'd like to make some suggestions for improvements.
Coming from Unity, I immediately noticed how similar they are. This is great because it makes the overall transition less complicated. However, Game Studio lacks some features or implements them in a very unattractive way.
1) The Asset View only supports Grid or a Thumbnail views. The Grid view is absolutely horrible, and not user friendly at all. It feels like it was built with the out of the box grid control from the very old WPF. I would remove that view completely and instead allow the thumbnail view to resize to a list similar to how it is done in unity. Also, the icons need some work. when they get smaller, it is impossible to see what they are. Again, this is something Unity does better in my opinion.
2) There needs to be away to dock the Game Preview window as its own tab. Right now, the only way to see the scene from the Camera perspective is to use the drop down menu and switch from Editor and Game Preview. This is very inconvenient because often times, you would want to drop assets into the editor and see how they look in the preview.
3) In Unity, you can hover over a text box (for example the transform) and the cursor changes to double headed arrow that allow you to drag the value while moving the mouse. In Xenko this feature is so buggy that the values jump up and down like crazy. Also there is a huge lag between the time it takes the editor to sync the changes therefore rendering this feature completely useless.
4) The solution explorer should act like Visual Studio's solution explorer. Right now, when I create folders and drop assets into those folders, the folder don't expand and show the content.
5) Right now every time I create a script I need to save the editor for the changes to apply and show on visual studio, and even than, it takes at least a min for them to sync up. It would be nice ,again if it would behave like in Unity - adding new scripts do not require to save the project.
That's all I have for now. I am sure I'll find more and will try to post them.
Overall it is a promising engine but I think a lot more work is required before it can be considered a competition.