The University of Virginia Sociology Department has recently completed and published a study outlining the "most important" factors of marital happiness, you can can read the entire report.

But I just wanted to include the summary bullets here, my comments are in red:

1. The most important determinant of women’s marital happiness is the emotional engagement of their husbands. Most research on American marriages has focused on the division of housework and paid work — who does what inside and outside the home. The new study, by contrast, shows wives care most about how affectionate and understanding their husbands are, and how much quality time they spend with their husbands. Emotional engagement comes from commitment (see commitment point below). But I have heard my wife express multitudes of times how much she loves the fact that I give her plenty of emotional support and feedback, making me "the best husband in the world" in her words.
   
2. Wilcox and Nock found that women are happiest in their marriages when their husbands earn 68 percent or more of the couple’s income. Likewise, women who do not work outside the home report happier marriages, and spend more quality time with their husbands. Let's pretend for example that we believe in evolution of humankind, males and females would have started out pretty much physically the same, or similar statures, like most cats, but for whatever reason, the males were more adventurous, they liked going out and finding things to kill to eat, they enjoyed the adrenaline, the excitement, the danger of the hunt and exploration was what brought satisfaction to the male psyche. After a few generations, it is not difficult to understand why our current physical statures emerged, the large, bulky musculature of males provided speed and power for killing food, the psychological ability to focus on a single issue and pursue it to finality, perfect for spotting a prey candidate and shutting out all other distractions and bringing down the meat. The soft, smooth, curvy features of females more adapted to bearing & nurturing children and engaging in less adventurous activities such as tending the fire & processing the daily meals, the psychological ability to scan a field and determine the best places to gather berries, grains, and other plant-based foods. Even if you don't believe in evolution, these same concepts are true, the only difference is that humankind appeared on earth with these physical and phsycological traits already fully intact, and the "traditional" family roles were created naturally as humans experienced the world around them. Finally, women who have more traditional attitudes — who believe, for instance, that women should take the lead in taking care of the home and family, and that men should take the lead in earning — are happier in their marriages, report more affection and understanding from their husbands, and spend more quality time with their husbands. The physical results of the evolution scenario are no longer necessary, but the original psychology that drove them into existence still exists, men prefer adventure and conquer, and are very good at pursuing individual tasks to completion; women prefer security, calm home-building activities and are exceptional at scanning the big picture and seeing the important pieces inside it.
   
3. Commitment also matters. Women who share with their husbands a strong commitment to lifelong marriage — who, for instance, think that marriage is a “lifetime relationship and should never be ended except under extreme circumstances” — are much more likely to report that they are happy in their marriages, and that they are happy with the affection and understanding they receive from their husbands, compared to women who do not have such a commitment to marriage. Shared commitment seems to generate mutual trust and higher levels of emotional investment on the part of husbands — both factors which promote marital happiness among women. Affection, closeness, and a strong sense of "team spirit" are the natural results of commitment. Take a company of soldiers for example, if they commit to each other (the whole company), they form a bond through hard work, loss, and close calls that remains unbroken even after decades of little or no contact. Hopefully, a family commitment never has to go through life threatening challenges, but as a husband-wife team work together to overcome the natural decay of the universe and build their life together using each other's strengths, that same bond will form.
   
4. Fairness does matter in shaping the quality and character of women’s marriages. Married women are happier in their marriages when they think housework is divided fairly and they perceive their marriage to be equitable. They also spend more quality time with their husbands when they think housework is divided fairly. But their perceptions of fairness in this area are not consistently related to a 50-50 division of housework. Almost two-thirds of married women report that they are happy with the division of housework; but most of these women also do the majority of chores around the home. Either because they have a traditional mindset or because they think their husband is making important contributions as a provider or as a father, they view an unequal division of chores around the home as fair. It is important to emphasize that "fairly" and "equally" are not the same thing in this case. Woman are not lazy, they do not wish to do exactly half of all the housework, then lie around waiting for the husband to come home from work and do his half. Women, like men, are happiest when they are anxiously engaged in doing good things. How much a woman is able to accomplish each day varies widely from woman to woman and is affected heavily by circumstances. Again, the traditional roles of housework have proven to me and my wife to be a conduit of happiness, the wife generally does the cleaning, cooking, and laundry because her threshold or minimum expectations for those are much higher than the husbands, likewise the man's ability to do "handyman" type work and yard work exceeds that of the woman's because of his threshold for the results. However, my wife and I have found that these roles do not imply exclusivity either, if I am lying around and there is some laundry to be done, I help out, and if there is handy-work or lawn care to be done and my wife has extra time, she helps me. We help each other at every opportunity, there is no "that's not my job" that is garbage, true unity in marriage comes from serving each other, and doing each other's job when possible, is the easiest and most effective form of service in the world.