simple: arrows just keep going over them, while activating anything that would normally be activated. in fact this could be the very "barrier tile" that we was once suggested. You can't go through it, but arrows and dynamites can. so basically: fragile, indestructible, steel crates = barrier tiles

so if we make fragile tiles a separate mode (I'm calling them modes now, not subclasses ), what about steel versus wooden crate? These would be the only two types of tiles. But this could help in organizational purposes in the level maker, I guess... I'm thinking maybe having three tabs in the tile list. One for wooden crates, one for steel, and another for other tiles, such as terrain, secrets, gems, power boxes, ladder, platforms.

as for the automatic recognition of an object's properties and settings, there has to be subtle differences in each tile. Currently we have the class defined by the material of the tile: Wood versus Steel. But for the modes, we need to come up with more differences. Some possible visual changes are:

shape of the tile - it doesn't have to be a perfect square. Maybe it can have dents on its sides? Or look like a hexagon?
color differences - for example, what if lethal tiles had a red or yellow tint to them?
brightness/darkness differences - what if objects that fall up are somewhat brighter, for example?
glowing  - maybe to make something look radioactive and more deadly?
visual marks such as cracks or FLAMES - or simply add more things to the graphics, such as dirt marks, or rust.
opacity - would probably work best for objects that fall up, since they are almost "ghostly" because they are so light that they fall up.

I personally think we should have 6 classes of tiles:
-Breakable crates (I try to distance myself from HatPC terms)
-Unbreakable crates
-Objects
-Enemies
-Non-solid (I need a better name for this)
-Scenery

Breakable are wooden crates, unbreakable metal, objects are anything else you can apply modes to (ladders, treasure chests, spikes, platforms, etc.), enemies are obvious, non-solids are anything you can't directly interact with, but still have an effect on the level itself (starting position, exit, water tap, water drain, etc.) and scenery objects are objects you can't interact with and which don't have any effect on the level. They are purely graphical.

As for the modes and visual differences, they could also get details to keep them apart from eachother. Like static crates could get metal holders around their corners, fragile crates could have cracks, indestructible crates could have some metal armour around it. Just a second image you paste over the first and over that you paste the image of the type of crate it is (arrow, dynamite).

sounds really good.