Status: This has been implemented for the next update. All objects show what kind of object they are next to them; steel terrain is also marked as such.
This suggestion was inspired by this post (http://www.lemmingsforums.net/index.php?topic=2489.0) and my own personal experience when creating levels.
It will be very helpful if the objects list has a label next to them indicating what type of object is. See the mockup picture below.
It should be also noted in the list that the locked exit on the Sega style is unlocked. This can create confusion between the regular exit and the locked exit.
You can also do the same thing to mark which is steel in the terrain list. I saw Gronkling's cyber tileset and wasn't sure if those yellow/black squares/rectangles were steel or not, and I had no idea that the grey cans in the Lab tileset were steel until I tested both tilesets.
I certainly like your idea of labelling each object, and I also appreciate how you picked up my recent post.
We could also label which terrain pieces are steel (Particularly the Martian set. I didn't realise in that set that there were two different types of terrain made of steel).
We'll see if this can be done.
Not entirely sure how easily the current editor can be modified to do this, but I'll look into it.
Found a way to do this, and am working on it. :)
EDIT: Done for next update.