In your list you didn't mention the topic of getting children. If you marry someone with the intention of spending the rest of your life together with them, I think you should be on the same page with regards to getting children before you marry.

What exactly do you think/hope will change between the current situation (which I assume involves you two living together) and the situation if you were to marry?

I don't know what is the significance of marriage for you, except symbolic. IMO the truly critical point is having kids. You probably want to have stable income before that.

Regarding things you're "just supposed to know": same thing happens to me with my wife. Haven't stopped us from being together for 10 years and raising a 4 year old son. Different people see things differently and have different assumptions on what is "obvious". The important thing is being mutually patient and forgiving (I know it's easier said than done, but it's doable).

Regarding the "extra feeling". Don't really know what to tell you. It is difficult to compare emotional experiences of different people. When our relationship started, it was mad, passionate infatuation. Now it's something calmer but it is obvious to me we love each other.

I had few relationships apart from my wife and virtually no serious relationships. Never bothered me.

Don't get married unless there is a compelling reason to do so. There's a base rate of 40-50% for divorce, and at least some proportion of existing marriages are unhealthy and unhappy. Divorce is one of the worst things that can happen to you, and many of the benefits of marriage to happiness are because happier people are more likely to get married in the first place.

She has said that she doesn't want to marry me if she's just my female best friend that I sleep with.

What are her feelings about you? Are you "just" her "male best friend that she sleeps with"? Your post comes across as rather asymmetric.

We both are concerned that I've not really had a relationship not with her, so there are no points of comparison for me to make.

Aren't you "both concerned" that she had too many relationships and so may decide that you are not for her precisely because she has these "points of comparison"? I suspect that she is the dominant partner in this relationship, possibly because she is more mentally mature, and this is often a warning flag.

She sometimes gets mad at me for things I'm "just supposed to know" to do, not do, say, or not say. I'm not sure if she's right and I'm a jerk.

Do you get mad at her for things she is just supposed to know to do, say or not say?

Anyway. DO NOT GET MARRIED YET until you figure out how to be an equal in this relationship (and if you think that you are, then you are fooling yourself).

As a starting point, run through this: http://www.justfourguys.com/female-divorce-risk-calculator/

Also, you should be the reluctant one, not her.

And, if neither of you are willing to live an at-least vaguely biblical marriage, then civilization would probably be better of with you just donating sperm to a sperm bank, keeping her from sleeping around, and encouraging her to marry someone who is stereotypically Christian and for whom she would be willing to convert.

She sometimes gets mad at me for things I'm "just supposed to know" to do, not do, say, or not say.

Well, you are. Pending more life experience, find the most un-politicallycorrect Game blogger you can stomach and start there.

Married guys make more on average, but the arrow of causality could point in either direction or come from something else.

And married women make less, so even assuming the arrow of causality is entirely from marital status to income it's not clear to me what would happen to your combined income.