Common question: What are the make it or break it issues in marriage?
To me, the foundational issues for marriage (for observant believers) are:
1 Aqida. It's the most important thing to Allah, and therefore it should be the most important thing to us. Beliefs set the framework for morals and laws, and therefore the very definition of "a good person" is rooted in aqida. Certain things in marriage should be the same (like core beliefs), while other things should actually be different in a complementary way (like temperment).
2 Purpose of life. It sounds abstract, and some people never think to ask themselves this. But it's critical. Some people just mosey along through life. That's most people actually. But if both sides are strong-minded and stronge-willed and have visions about the world and how they want to affect it, then these visions should not clash.
3 Adaab and akhlaaq. For example, mingling with the opposite gender can be a serious issue in a marriage. Those things need to come up in discussions. The way people talk and the kind of humor they have can make or break a relationship. Cultural compatibility can fall under this category to a degree.
4 Schooling philosophy for kids. Big issue. Islamic school, home school, public school. People can change their opinions over time, but you can only work with what you have now in front of you at the moment, and both should be on the same page with this.
5 Where to live. Some people have these ideas about moving 'back home' or doing hijra, while the other foresees themselves living next to their mom. Big enough issue to warrant discussing. Even more important: will we live with parents in the same home or not. In some cultures, this is a norm, while in others it is not and can be a cause of frustration and argumentation.
6 Money. It should be known that the husband needs to carry the load. What the wife contributes is her sadaqa. But he shouldn't expect it or demand it. At the same time, standards of living need to be known. There should be no surprises here.
7 Working hours should be known. A man may have no problem with his wife working. Another man may not want that. This should be discussed. There should be no surprises here. Likewise, some jobs have very odd hours, or long hours, and a woman might not want a husband who only pops in for dinner and sleep.
If you're in talks with someone and you agree on these seven points, while at the same time not feeling repelled by their personality or look, then that's really it. It's not every day that that all these things line up. To break it off based on something outside these fundamentals doesn't make sense.
I think things like 'chemistry' are over-rated. People grow on one another. And the Quran tells us, "...spouses to live with, and He placed between you love and mercy." Thus, by living with one another and having unity on the major issues of life, love grows. In contrast, two 'compatibe' people whose views on life and the world are polar opposites will not last.
To a degree, alot of this is opinion based, so feel free to add your own two cents in the comments.