Bedroom dressers - the bottoms fall out of the drawers and should ideally be glued in place (the instructions don't suggest this). Even with glue, they sag after a year or so.
Kitchenware - their handles on their pots and pans are very heat conductive so make sure you're wearing an oven mitt. Their kitchen block furniture with the thick wood top is well made, though it should be stained and varnished to avoid stains.
Chairs - maybe half the chairs I've gotten from Ikea had poorly drilled screw holes such that the chair couldn't be assembled correctly. Fortunately, the benches are made of such soft pine (read: scratchable) that you can drill your own hole easily.
Sometimes Ikea will eliminate a part (eg: a supporting brace for a dresser) but not redesign the assembly instructions. You might think you're missing that part, when in fact it was omitted deliberately.
They save money where they can by selling you disposable furniture.