Mr. Teapot
First Post
4e is a different beast. In 3.5 challenges were measured at a 4 on 1 basis, meaning every action by a monster had a lot of importance. Burning one of those to become invisible was a high cost that one PC could negate through several means and ready actions could interrupt a monsters careful plans.
Not just that part of the economy of actions, but the other part is even worse: against a single creature, if it turns invisible, then there are five PCs sitting around wondering where it went and unable to do anything cool. Against five creatures, if one turns invisible you just attack whichever others are visible. (This is a concern when designing solo lurkers in 4e, too, of course.)
I don't think the quasit's invisibility is an issue, because it has four things it wants to do each round (bite, evil temptation, invisibility and move) but can only do three each round. Trying to find a turn where it can bite/daze and then turn invisible and move will be tricky because of reach 0. Any other situation means that the PCs know where the quasit is at least.