Friends, Lovers, or Nothing?

(I hate John Mayer as a person, but I love his music.)

So my friend F came to me today with a question that I have pondered before and will probably ponder again: What exactly is the difference between very good friends with benefits and a relationship? Very good friends meaning you text each other regularly and can talk about things and spend time together outside of one another's bedrooms [disregard the fact that such a situation may not be possible]. 
This relates back to everything I've ever wondered about how to know whether you like someone enough to be in a relationship with them. My friend and former roommate J noted very observantly that I have a tendency to closely befriend guys who fall into my range of "date-ability". I like to surround myself with people who aren't assholes, yeah. And who are smart and funny and whom I can have good conversations with...anyway, this is getting off subject. The important part is, if any of the guys I let into my innermost of circles expressed romantic interest in me, my panties would be off in a second I'd be totally down for that. They've all been pre-screened. They've probably all passed most of my tests. They're good guys. ...But perhaps it shouldn't be so easy for me to cross the line between how I want someone in my life. [Okay, well actually I'd probably be so terrified at the prospect of losing our friendship is shit went sour that I wouldn't want to risk anything by changing things. Because sometimes I'm a pussy.]
A second part of F's question was what's the difference between spending time with someone as a [very close] friend and spending time with someone in the context of a relationship? Is it just the intimacy? I once heard someone describe a relationship as just having someone you know you can hook up with every night, and I don't agree with that. A) Something comes from being emotionally intimate with the person you're being physically intimate with. B) I feel like the person you're in a relationship with all of a sudden becomes your best friend. They start to take on (or at least share) roles your best friends used to claim: the person you eat meals with, the person you text when someone awesome/terrible happens, the person you want to do something with when you're bored, the person you go to events with...but something is different about these things now. Now you're there WITH someone, as opposed to with some people. You and the person you're in a relationship with belong to one another in a way your besties never will. C) Part of that mutual ownership thing is sharing more things about yourself. The person you're in a relationship with has gets to see you with no makeup on and with bedhead and morning breath and explore all the parts of your body even when you haven't shaved. They get to know what your face looks like in moments of extreme pleasure, and also get to calm you down out of panic mode when crazy shit happens, and to comfort you when you've got killer cramps. They kind of take care of you and you kind of take care of them in a more full-time way than besties. D) Relationships are about mutually prioritizing another person, and they are exercises in reciprocity in a way that is not demanded by friendship. E) At least in my experience, you're still not guaranteed to get it every night (or even every night you want it). :\ 
But should there be some way I initially like hypothetical relationship X person that is different than the way I like my closest guy friends? A "spark", if you will? Should time that we spend not physically touching one another feel different than time spent hanging out with a friend? I feel like the answers to these questions might be yes, and given that, I feel like I might be doing things wrong. Except I can't figure out why. People say their spouses are like their best friends all the time, don't they? It's generally considered to be cute and appropriate and damn near ideal. So what separates a friend from a friend you can casually fuck with from time to time from a friend who could also be your life partner? Is something missing from the equation if an appropriate way to describe my relationship with a hypothetical boyfriend is that he's like my best friend AND he sexes me so good! That kind of seems like exactly what I want in life...
K once told me that fwb don't spend the night after they hook up. [Neither did my ex always. Neither did my ex before him ever, but that was high school...] I would like to argue that fwb don't CARE, but that also seems kind of false, if they are decent friends. In fact, isn't that usually the downfall of fwb relationships, that one of the two parties catches feelings? So I think it comes down to the fact that fwb is a relationship of convenience, whereas a real relationship is a matter of cultivating and prioritizing and reciprocity and choice and consequence and sacrifice and real hard work. And fwb will run itself into the ground even faster if you try to do relationship [or friendship?] type things like go out to dinner or catch a movie, right? Can you really be a friend with a fwb?
I suppose the underlying question in all of this is, are relationships worth it [even considering all the pain they can bring]? I think that feeling that you are the most special person in someone's life is priceless. I think being taken care of physically and emotionally and in whatever other ways you might need all by one person and being able to take care of that one person in the same ways is one of the greatest joys in life. I think you get out of relationships what you put into them, and so if your attitude is that your significant other is this cool person you're sleeping with on the regular and hanging out with too...that's not all that significant, is it? Best-friend-ship is probably the most significant relationship I can conceptualize at this age, and so I'm aiming to transcend that in my relationships. Mostly with the little intimate things that my friends and I will never experience together...and by getting naked. XD