I read this book back in May, 2005, as part of my Gothic Lit. class. It's not a book I'd read again strictly for pleasure, but there is a strange quality to it that beckons me to read it again.
While a fairly absurd and not-very-frightening book (at least to modern readers), this book is worth reading as it seems to contain every element that is a staple of gothic fiction -- and why not? It's the first, after all.
After the class and a little thought, I lean toward considering the following elements to be the staples of "true" gothic stories:
1. Numinous (frightening and awe-inspiring) supernatural elements (one could say that should be drawn loosely from real-world beliefs, but I won't make that stipulation myself)
2. Excessive violence (not necessarily blood/guts/gore, but something that leaves you thinking "that wasn't called for")
3. Sexual perversion (not necessarily anything explicit, just hints at something "not right" -- this element makes things both more exciting and more menacing)
4. Madness
5. Helpless hero (necessarily useless, but overwhelmed, unable to accomplish everything and/or take an active approach to the problem)
6. Social injustice (a challenge to "life as usual")
6. Religion gone wrong (a bleaker, maybe questioning look at religion and religious beliefs)
The surprising thing is that it does this while remaining a fairly tame book. It's excessive violence is performed off-camera, as does the majority of its supernatural elements. Manfred's desire to leave his wife on the basis that their marriage is actually incestuous in order to marry his late son's fiance was sufficiently disturbing to me but far even from X-rated. Manfred is flighty and prone to a kind of mania. The hero is vastly overwhelmed, stays on the defense, and is unable to save the one thing most important to him. At the heart of the novel are pointed social and religious questions/commentary.
One of the things that has fascinated me with this book is the retellings it has inspired in The Old English Baron and The Castles of Athlin & Dunbayne. Both of those are significantly less gothic than Otranto (especially Castles, which is not gothic at all), but are better retellings of the core romance between the hero and his love.
All in all, I'd recommend this work to anyone interested in gothic literature. I'd also recommend The Old English Baron and The Castles of Athlin & Dunbayne (especially the latter) as better retellings of the romance in the book.