This feature already exists! You have the option of marking questions as favourites. To do this, simply click the star below the voting buttons of a question as shown in the image below:
In the left-most picture, the star is yellow because I starred that question for myself as a favourite. I can also see that 14 other users starred that question. The middle image shows a question favourited by one other user (but not me) and the one on the right wasn’t favourited by anybody.
These favourite questions show up in your profile under the favorites tab:
Again you can see that fourteen other people favourited that first question which you saw before. It also includes all other questions I ever favourited. If something happened to one of those that renders it active in any way, there is a small blue number next to favorites which signifies how many active favourites there are; these are then highlighted yellow in the tab. This helps you keep track of what changed within your favourites.
There is no limit to the number of favorites you pick. Furthermore, you can retract a favorite at any time by just clicking on the star. There is no limit to the number of times you can retract or reinstate a question as your favorite. (source)
Of course, while there is no limit you have to decide for yourself when this feature loses its usefulness.