There are two ways you can see (at least in your inventory) how much you have of a certain item. One is while hand crafting it will show the total, the other is when picking up a bunch the center of the screen shows it for a short time.
I'd like some kind of way to relatively quickly see how much of a specific is there in my inventory or a container.
An example of how that could be is on the popup when you hover over an item it says how many total you have in that container this item is in, along with the already existing information. Or on mouse over alone (no popup) somewhere a counter that shows it. Again, just examples of how this could be implemented to show what I mean.
The todo list is great and I constantly use it, but that is not what I'm refering to. A usage example is that you stash items for a milestone or you calculated how much you need for a specific project and want to check if you got that.