Add arrows (or equivalent) to stairs and elevators
In the previous (Legacy) version, you could go up and down stairs or elevators by clicking on an arrow. This feature was both fun (you can walk in the building) and useful (to check that levels you just added to OSM are correct). For some reason that feature has been removed (or I can't find it).
Now you need to switch to Advanced mode (since it is not the default), click on the object, check its "level" tag (in the left window), then click on the appropriate level in the level list (on the right part of the screen).
Just to let you know that I still use the Legacy version and hope it won't disappear :-)