One of the most popular sayings of Prophet Muhammad (peace be upon him) with regards to choosing a partner is:
"A woman is wed for four reasons; her wealth, family status, beauty and religion. So, pick the one with religion or you will be at the losing end."
It is also quoted from Prophet Muhammad (peace be upon him):
"Marry the loving and child bearing woman, for I shall out strip other nations with your numbers on the Day of Judgement."
I do believe that these two hadiths apply to both husband and wife, as a man can't be looking for a religious wife when he himself is not religious, nor can a fierce man be looking for a loving wife as this is a great injustice to the wife should she be forced into such marriage (though technically; in Islam a woman can't be forced into marriage).
This is further reinforced by Allah's words in Surah An-Nur:
"And the sinful women are for sinful men, and the sinful men are for the sinful women, and the virtuous women are for the virtuous men, and the virtuous men are for the virtuous women."
Therefor, qualities of a good life partner should include:
This is the first and most important trait of all. As this will ensure that the family that you are trying to build is one that practices Islam and that conflicts between the couple with regards to what is permissible (halal) and what is not (haram) is minimized.
Our Prophet Muhammad (peace be upon him) also mentioned that:
"If someone whose piety and character satisfy you, marry him, or there will be disorder on earth and great deal of evil."
What can be understood from this hadith is that; should you choose a non pious partner over a pious partner, there is a possibility that your off springs will not be pious due to the lack of guidance and role model at home.
The pious person, should he or she fails to secure a partner; might misbehave and the idea that "the good guys finish last" will propagate among society.
2. Family Status
The second quality should be family status and background. The family does not have to be wealthy or of high status, but you should ensure that they come from a good background as it forms a part of your partner's upbringing and he / she will be more or less influenced by it. You might also want to consider a family with similar background as yours to facilitate compatibility and minimize misunderstanding between in-laws.
A man from a less fortunate background might find it difficult to adjust and respond to demands from the wife's family of differing background and this might create strains on the couple's relationship if the wife's family is not understanding or vice versa.
3. Wealth and Beauty
As for the other qualities of wealth and beauty, it is subjective to each individual. It is fortunate if all four qualities are in your chosen partner, but if otherwise, it is best to choose religion and family background above other qualities. Remember that beauty can fade and wealth can be acquired with hard work and Allah's grace and it will only be beneficial to you should you spend it in Allah's ways.
4. Affection and Fertility
Fertility is a factor of consideration, as the purpose of marriage in addition to completing a person and bringing peace in our hearts; is to produce off springs to ensure the continuity of the Muslim society to spread the word of Islam to the world.
Remember, even if we are not active in da'wah and and spreading the word of Allah in our community, our continued presence give them a glimpse of what Islam is. Our honest and virtuous acts within our community will at some level attract their interest and at the very least make our community familiar with Islam and Muslims.
Affection will then ensure that the romance between partners stay alive throughout the lifetime that you are going to spend together and make the disagreements that you are most certainly going to encounter easier to resolve.
Affection will also assure that your children will have a happy home to grow up in and a positive environment at home is greatly beneficial to the development of any child physically and mentally.
5. Other Qualities
I am sure there are other qualities that you look for in your partner and I think these qualities are highly subjective. I should think that you would try to find qualities that complement yours in order to complete you. But please bear in mind that no one is perfect and in screening our would be partner's qualities, we should remember to screen ourselves too.