You Can't Read Books or Cast Spells or Use Weapons

The sunlight is a big thing It's definitely a very satisfying mechanic I'd say the other big idea is like you said you are fighting vampires, but you also are afflicted with vampirism. You can't just choose one and and go through the whole game as it you have to swap back and forth at different times. There may be a level that's much easier as a human you need to be human for that or one that's much harder as a vampire. They're completely different play styles.