Overall a great job! I wrote down my thoughts as I was playing, then came over here and realized that it was almost exactly the same as HiImBarney's review. So I'll second that, and add a couple things.
The lateral movement vs diagonal conveyor belts didn't bother me at all. I thought it worked well with 8-way movement.
There were moments where it would be unclear whether the item I was entering was registered, like if the player model was at a shallow angle to the box, or if the box would go behind a pillar, etc. I think either removing the ability to drop items on the floor, or adding some more visual feedback on placing items would help a bit.
I was having trouble cycling through the inventory, remembering which way was up and down. If the mouse wheel is a usable alternative as HilmBarney alluded to, that's great. Personally I didn't realize that. Perhaps I missed it in the controls screen. But I would remove the use of x and z if possible, or at least make them swappable, like invert-y-axis is in many games.
Again, great job, great game, and good luck in the voting!