Why is the Slytherin common room so unappealing?
I think that each house’s common room is meant to be appealing to the students in said house.
Take the Ravenclaw common room for example:
A wide, airy room filled with multiple bookshelves and tables would be the perfect place for your average Ravenclaw to be in. It just screams intelligence and creativity. Would a Gryfinndor find this layout appealing? Probably not, which makes sense, because the rooms are built around the values of Ravenclaw, not Gryffindor.
Going back to the Slytherin common room:
This place would be much more appealing to Slytherins than to any of the other houses because it’s built around the core Slytherin idea of ambition. I mean, just look at the place! With its greenish hue, lanterns, and fancy tapestries, it definitely is one of the grander of common rooms. Unlike the other common rooms which try to provide a homely feel, this common room gives off a more royal kind of feel, which is excellent for an ambitious little Slytherin. Plus, it has windows that look into the lake, which makes it even more impressive.
The common room is most appealing towards the members of its house and while you might find a certain common room unappealing, members of its house would find it perfect because it’s centered around the core values of the house. I personally find the Slytherin common room super appealing, especially the whole “windows that look into the lake” thing, but I suppose that's because I'm a Slytherin myself ;)