I know this is behind the times. Many of our generation is staying single, cohabiting, or postponing marriage, but as a married woman with a couple of years behind me, I have a few things to say. So for those of you considering marriage on any level, and even those of you considering entering a long-term relationship even without a marriage commitment, this is for you. I'll try to speak mostly from a social standpoint, considering that religion is not everyone's gold standard. And even if you are highly religious, you should be heavily concerned with evaluating a potential spouse or partner from a social, objective standpoint in addition to considering your religious beliefs.
The no-brainer is that you should be in love. As we live in a first-world country, we mostly have the luxury of marrying for love. But just being in love does not cut it. Think of all your relationships where you loved a person, but there were things that happened that cut into the happiness in the relationship, or you were otherwise incompatible even though you loved them. I'm sure many of you could instantly name an ex-boyfriend or ex-girlfriend. For those who can't, I'm going to be frank. You can love someone so much, and still it would be an extremely bad idea to continue dating them or to consider marrying them. Don't be stupid in love, and take note of the rest of these considerations.
You should be on a similar maturity level. I'm not saying you both have to perfectly and utterly responsible. We all have room to grow, and there are things about each and every one of us that could use a little polishing. Take a look at how your partner lives, especially and even if you do not live with them. Do they at least attempt to keep house? Someone with a really busy life may have less time to clean and keep things tidy, but you can evaluate on an individual basis. If your prospective partner works very long or late hours, take that into consideration. If they do not work long hours, do they keep the house from looking like a tornado hit it? Do they cook a meal or two occasionally? Does the items they buy, the things they do, and the lifestyle they live appear compatible with their pay grade? If they don't work many hours, but they are constantly buying new toys, gadgets, or clothes, then they may be using their money irresponsibly. If there are piles of junk everywhere, a kitchen sink overflowing with dishes, or a bathroom in desperate need of being cleaned, these may be red flags that this person does not spend whatever time they have wisely.
You should agree politically. I once heard that political affiliation is the greatest factor in whether or not a couple will survive. It makes sense, when you think about it. Your political opinions are formed from the core of what you believe, what you've been taught, and what you think about the world in general. If you don't agree at the core, how do you expect to agree on matters that are more trivial? If you consider yourself an activist, constantly petitioning, discussing politics, etc., and your partner is so politically apathetic that they refuse to vote at all, you will likely do some butting of heads. Definitely consider your partner's family in all of this. It will depend on how close they are to their family, but if you agree with your partner's point of view and it differs greatly from the rest of their family, it may be a lot harder to get family support from them.
You should both be willing to work on whatever you need to weather the storms of a long-term relationship. Even the most loving of couples will have bumps in the road, but the important part is how well you can handle them. If you and your partner have a hard time working through little arguments, or there are a lot of silent treatments, screaming fits, or any kind of abuse, emotional or physical, it is not going to last. It will drag out forever and make you miserable until you finally give up. Get out while you can, especially in the case of abuse. If your partner physically harms you or manipulates you into getting what they want, you are in danger of losing yourself, and the abuse will not get better with time -- only worse.
You should know that you want to marry and/or be with this person for a long, long time, even until death. This seems like a no-brainer, but it's not. I once had a coworker ask me if I'd known my husband was "the one" when I married him, specifically because she did not feel that her husband was "the one." Now, "the one" is a vague and fluid concept. Some people believe that we have a soul mate, only one person who can truly satisfy the desires of our heart. Others believe that we could love and get along with many people, but that the choice is what is most important in the end. I am of the latter school of thought, but even so, I knew the man I was marrying was infinitely better than any man before him. I knew that I wanted to marry him, and I also had no doubts that I would be happy with him. This isn't fail-proof, as people all change. But if I had any doubts whatsoever about marrying this man, I wouldn't have gone through with it, as should anyone in the same situation.
Picking a spouse seems easy when we are young. All we have to do is fall in love, marry that person we love so dearly, and live happily ever after. It isn't that simple. Please, please, please...I beg you to consider your future marriage or partnership with adult seriousness, especially if you are young, and even if you consider yourself mature. There is more at stake than you realize when you pick someone to marry. Make it a good choice.