Ενδιαφέρον is indeed a noun here (interest), and it is neuter in gender. In order for you to directly translate "This conversation is interesting", you'd have to use the feminine adjective form, ενδιαφέρουσα (ο ενδιαφέρων, η ενδιαφέρουσα, το ενδιαφέρον). And of course, it would be perfectly correct.
Both sentences have the exact same meaning though. ^.^
Sorry to confound this further but I disagree that "had interest" is a good translation. In English we might say that a conversation had interest FOR ME or FOR SOMEONE ELSE but we wouldn't just say "a conversation had interest." It puts a slight emphasis on the person who found it interesting, whereas "the conversation was interesting" implies that anyone would find it interesting and no need to say who. It's a very subtle distinction, and I'm not suggesting you remove "had interest" as an option since its the literal translation and its certainly not wrong, but it's not totally accurate either. Hate to be picky but it seemed useful to mention.