1) He's good at his job
Nobody wants to go out with an incompetent bumbler. Confidence and competence are the ultimate aphrodisiacs. Ladies want someone who's good at what they do. Someone who can inspire his followers and push the boundaries of his chosen field. As the most powerful, most feared dark wizard of all time, with thousands of Death Eaters hanging off his every word, Voldemort fits the bill perfectly.
2) He speaks multiple languagesAs the world becomes more an increasingly global community, the ability to speak a second language is imperative. Not to mention a great way to romance the ladies. And as far as the language of love goes, Parseltongue is the new French.
3) He knows how to use his wand
Everyone's looking for someone who makes them feel that special spark. As the world's premiere sorceror, Voldemort can make you feel things you've never felt before. Like extreme pain or the strange tingling sensation you feel a second before being turned inside out.
4) He's passionate about his hobbies
There's nothing worse than dating a one-note wonder, somebody who has no interests and will bore you to death over dinner with pointless anecdotes about that hilarious time they filled out Form 27-B incorrectly. Modern women want to date a compelling man who has a range of interests. As well as taking over the world, Lord Voldemort's hobbies include sudoku, meeting Robert Downey Jnr. and killing Harry Potter in as many horrible ways as possible.
5) He plans aheadSome men never think about the future. Happy to just cruise through life, these guys never put anything away for a rainy day, or take out the home insurance you've been bugging them for weeks about. And then one day they're gone and you're left with nothing. Voldemort, on the other hand, always has an eye on the horizon. Between listening to prophecies and making seven Horcruxes to ensure his own immortality, the Dark Lord will always be there for you. Whether you like it or not.
6) He's not big on appearances
Let's face it, most men are sexist pigs. They don't care how many degrees you have, or that you revolutionised scientific thought by formulating a widely accepted, consistent framework for unified field theory. They only care about how you look naked. Lord Voldemort, on the other hand, is a sensitive new age guy who doesn't care about appearances. This is clearly demonstrated by the fact that he hangs out with Helena Bonham Carter. When you have no nose or hair and wear a robe everywhere, you're pretty much guaranteed to be above such petty concerns.
7) He knows what women want
Literally. As a skilled Legilimens, the Dark Artist formerly known as Tom Riddle can read your mind, plucking out your deepest, darkest desires. And then use them against you to further his plans for world domination.
Don't even pretend you're not interested, ladies.