Theming stylisation order isn't respected in a layer
At the moment you can set the display order for a layer but not within a layer e.g. a thematic.
Let's say there is a set of 20 busroutes, each indicated by different widths and colours. You can move the order in the legend up and down, but not the order of data display. The data is displayed in the order it comes out of the datasource, and you get random results. A thick line may overlay a thin line where there are shared busroutes. This means you have to set up 20 layers to get the desired results, cluttering up the user interface, and slowing down the system by doing 20 queries instead of 1 (significant when you have 200+ layers already)
Being able to manipulate the order of data display witin a thematic would be very powerful.