It's important to note that ArchiCAD uses layers differently than AutoCAD. Layers are entirely used to hide or display the object, but do not control anything else.
For line-weight, they're controlled through the use of pen sets which act a lot like layers except they don't control visibility. You can sort of think of AutoCAD layers being a combination of an ArchiCAD layer and an ArchiCAD pen set. This allows for flexible control over how elements are displayed, but does cause a bit of confusion for folks switching from AutoCAD.
This can ease collaboration since some disciplines have different requirements for lineweight than others. For example the Architects in my office use pen 37 for interior walls. In their drawings, they're using color and a heavy lineweight as defined in the pen set they created for their drawings. In my drawings however I require grayscale and hairline for walls since they're a secondary feature. Rather than having to move all the walls from one layer to another and incurring the wrath of an Architect who's just had all the interior walls disappear, I change the pen set of my view and the walls look right since the pen set decides how pen 37 is displayed, not the layer or some hard-coded part the object.
Oh, and you can search for objects by layer (and/or pen) using the "Find & Select..." tool in your edit menu.
