Letterboxd.com has an effective and visually pleasing, unobtrusive way of distinguishing watched movies. There's a setting called "Fade watched films" that fades (darkens) the cover art so that only the unwatched ones stand out. When you hover over the faded movie, the cover art is shown is full brightness. I would love something similar to this to replace the current watched indicator design in Infuse.
See attachment for screenshot of implementation in Letterboxd.