1. The most important thing is middos.
I know you've already heard it. I'm sure you would readily agree. But when you go out, and she is really pretty and funny and interesting, are you willing to look past how she talked about one of her "friends?" Or her lack of compassion for the girl who her friends were making fun of? It can be easy to lose sight of what's important when you are on a date Make sure you are honest with yourself and not making excuses for the other person's behavior because of something frivolous that you like. A person's middos are going to effect every day of your life. Just because she's the only girl you ever met who knows who Ron Paul is and loves Led Zepplin, this isn't going to effect your daily life all that much. I'm not saying common interests are irrelevant, but not as much as middos. Additionally R' Pam says that children receive their middos from their mother, in the same way that wine acquires the taste of the barrel it was fermented in. And nobody wants their kids to be a bunch of jerks.
2. Looks aren't irrelevant either.
R' Pam asks a question: Why does the Torah emphasize the beauty of various biblical women (Imahos, Esther, Chava, Avigayil...), but then in Eishes Chayil we all know "sheker hachein, v'hevel hayofi"? Is it important or not? He answers that beauty is a bunch of zeros. If you don't have the One in front of it, it's worthless, but if its got the one (which in this case is Yiras Hashem), it adds value. Obviously you have to be attracted to a spouse. How attractive? Now thats tough. I can tell you it's somewhere between supermodel and utterly repulsive. One of my Rabbeim says "she has to be nice to look at."
As a guy I can tell you that this is definitely something I struggle with. There have been times that the girl would walk out on the first date and my heart would completely drop because I didn't find a girl attractive. More for the struggle with my decision that I know is going to happen. That's honestly the hardest thing for me, and in some ways the worst part of shidduchim. I hear that people's looks can grow on you, but I don't want to go out on more dates than I have to. I also have a hard time giving a fair chance. I haven't really broken anything off because of looks, officially. I usually try and find something less superficial, but sometimes I know if she looked like a supermodel, I could live with my supposed reason for saying no.
When I get to the door and a girl is less than attractive I feel terribly guilty. I know looks have nothing to do with the person. The real person. But it's there. I'm always left wondering if I'm being too picky. You hear the stories about the people who won't settle for anything less than a gorgeous girl, and they end up alone. I'll be honest-I want an attractive wife. When I picture my dream girl, I think of someone who is funny, smart, frum but normal, and attractive. And when a girl is even just okay looking, in the back of my head I'm thinking am I really ready to be married to this girl for the rest of my life? Do I really need to "settle" for anything less than a pretty girl? I wish my mind didn't think that way but focused on middos and hashkafa.
I think this is the toughest thing for everybody in shidduchim. There is no clear line, no measuring stick, no scale you can use to see if this is your bashert. How do you know when you are settling for something not for you or you're just being picky? Did I just say "no" to my zivug? Or did I say something that I shouldn't have said causing my zivug to break up with me? It would be great after every dating experience Hashem sent you a text "Nope that wasnt the one. Try again!" I wouldn't think shidduchim would be all that bad then. You would know that you're going to get there, you just need to keep on trucking. We just need to have more emunah. A friend once suggested to me to have more kavanah during atah chonein for shidduch decision-making. I try.
This is our avodah. It's not easy. But we try and do our best. We move on, always looking backwards, wondering if we have made the right decisions. Hopefully we have. And I wish you that you won't have any more doubts or questions. And if you do I hope you'll come out of it stronger for it.
P.S. I do think a big part of the shidduch crisis that isn't addressed as much, is the guys' unrealistic expectations of what a girl should look like. Our generation in America, no matter how frum you are, has been exposed to unrealistic, photo-shopped billboards, movies, and ads. We think we deserve a girl who looks like that. I'm just as guilty of this as anybody else. How do we get past it though?
I know a man needs to see the girl before the wedding halachically but not girls because they aren't as shallow. But I'm curious to hear how much girls struggle with looks.