2. Gore
What does nearly every modern horror film have in common? Massive gore - and quite often the gore is a substitute for true horror. Now obviously gore has a rightful place in a horror movie and frankly I can't imagine a horror film without gore anymore. Gore is even used by Hitchcock to some degree, but it is never the main focus of his films.
Whenever he uses blood, Hitchcock tends to position it for more dramatic effect, such as in
Psycho where the blood (chocolate sauce) mixes with water and swirls down the drain. That scene with Janet Leigh's life fluid circling the shower drain is one of the most iconic horror images of all time.
3. Simplicity of Plot
Hitchcock's horror films have an actual plot with developing characters, back stories, and relationships. With a film like
The Birds, the plot would be summarized as "A socialite seeks to flirt with a lawyer by traveling to his weekend home where she meets his mother, sister, and former lover. Upon her arrival, the socialite and all the townspeople are attacked by successively larger and more violent waves of birds until everyone is forced to leave the town.". But one can easily look even deeper into
The Birds. For example, there is the character of Annie Hayworth (portrayed perfectly by Susanne Pleshette). Annie moved to Bodega Bay to be near our hero, Mitch Brenner. Unfortunately for Annie, Mitch's mother got in the way effectively ruining the relationship between Annie and Mitch. But Annie hasn't given up. Along comes Melanie Daniels bearing a gift of birds for Mitch's sister. Annie meets Melanie, and in her Annie recognizes the beginning of the end of her relationship with Mitch. This subplot, wherein Annie's relationship is doomed, foreshadows the eventual death of Annie at the hands, or rather the beaks, of the psychotic birds who arrive in town on the same day as Melanie.
Contrast this with, say,
Friday the 13th (one of my favorites), the plot for which can be summarized as "A bunch of kids head to the woods for sex where a killer gruesomely slaughters them one by one until the final teen escapes death.". Now of course there are minor relationship plots in
Friday the 13th, such as Jason and his mother, but nothing with the depth of
The Birds.