Interesting is not an unconditioned and objective judgment. A work of fiction may be interesting to some people and not interesting to other people. Thus if you want to write about what makes Silas Marner interesting to you, you need to think about what makes you keep reading the book once you have started it.
As Eliot's prose style is complex and sophisticated, using a heavily philosophical vocabulary, the putative audience of the book is one that is well-educated, intelligent, and intellectually curious; Eliot is not attempting to reach the same audience as the young adult fiction of Rowling or other popular bestsellers. For such an audience, one point of interest is the close analysis of the character of Silas, and how he progresses from despair and retreat from society to a sort of redemption, not due to hoarding gold, but due to the need to take care of a person even more frail and vulnerable than himself.
Another area of interest to many readers is Eliot's analysis of evangelical religion and the way both the good and the bad of Marner's character and experience can be said to stem from his experiences with both organized religion and personal faith.