>>28741
The main timesink depends on genre of the game and scope of the project.
Sometimes it can be level design (including dialogues etc.), code optimisation or anything else really (engines used to be troublesome in the past as well, as many upstarts had to make their own or rework some old ones).
Its usually 3D assets tho. I still remember the conversion from prerendered 2D to 3D boning rpg's for some 6 years, putting an extra pressure on the budgets, rapidly reshaping dev teams and dropping the quality of the gameplay and storytelling into the toilet as publishers rallied to divert all resources and attention to graphics. I'm still waiting for my real Fallout 3.