I think it depends very much on the characters of the people in the marriage. Their ability to work together as a couple, get over difficulties together and who progress together is very important in my opinion.
Talking about children, parenting ideals (because you don't know what your actual STYLE is or how you will both be as parents until you've had the children, surely?) money management, how you deal with your anger/temper/bad days, are all things you should talk about or think about together before you walk up the aisle.
I think if there are huge red flags before marriage, like violent arguments, not living together, very different backgrounds that aren't able to be overcome, is where the issues lie.
But for people like me and my husband, who began a relationship at 18, married at 23 and are now approaching 30 it's hard to pin it all down really. We're lucky enough to have had similar upbringings and the same basic values. We've both grown and changed a lot over the 11 years of our relationship, but we've managed to rub along together well. We have sort of similar money ideals for example. I like to save a lot and worry about having enough in the bank. Husband likes to be able to buy himself things, but is sensible enough to save money too and so I can deal with that because it's not like he is doing it to spite me. We understand each other's concerns over money. We sort of discuss how we think we will be, or idealise parenthood, but we'll have to learn about it together when it eventually happens.
One of our biggest strengths is simply that we care about each other. If you don't really care about your spouse, their opinions, feelings etc, how can you possibly have a successful marriage or relationship?