Nothing more than a Doom clone really.
It's as simple as that. All the bosses, enemines, weapons, are in a sense the exact same as Doom and are only different in apperance. Though it was a big hit for many, I for one have never been a big fan of the whole fantasy genre so I guess my feelings on it were quite biased. I guess I just expected more considering how Doom was such a revolutionary game, in terms of gameplay and graphics. Heretic didn't improve on either, and in many places the graphics seemed worse than Doom. I remember playing this game shortly after it came out but I really don't remember much of the game at all because it just really didn't appeal to me. I do remember only playing it to finish it because I hate not playing a game all the way through. I also remember being happy when I was done with it so I wouldn't feel obligated to play it anymore and I went right back to my beloved Doom :P