Oh I love NL - basically every aspect (tho I have to say it's awfully easy to keep sims always platinum - just take 'em on a date/outing where all the wants revolve around the date aspects or things dt and roll 'em over again and again).
I don't think I have all the romance subtleties figured out yet at all. I'm not at all sure what makes them *really* like each other. For example, I created a couple as adults - gave them turn ons that match (well more or less - one of his is swimsuits so he likes her better sometimes than others) and made them fairly compatability personality-wise (I thought

So sure enough they come up with 2 lightning bolts, 3 when she's in a swimsuit (I've never seen more than 3 - tho I've heard reports of 4), both have have a baby wants (which is what I wanted - I believe he's pleasure and she's popularity iirc). Anyway so I think everything is going great...
She gets little hearts all over the place every time he goes by - but he doesn't for her. Not even if she's in a swimsuit. He brings a female friend home from work and I can tell he's into her big, although there are no lightning bolts on their relationship score, and he keeps thinking about being married when he looks at her or she talks to him (the wedding ring shows up - I thought this was way cute. He has to remind himself he's married). If I click on her to choose an action, the ones with lighting bolts are there (flirts and all) - so I gather some flirt-type interaction is required before the lighting bolts show in the relationship score, at least if the sim is married/attached (I think this is a good thing - less "buggy"

So I have him scope the room - both the work friend and his wife are there and he comes up with "new friend is HOT" (NOT, needless to say, most attracted to his wife). So I do the flirt/checkout interaction, figuring there must be at least 3 bolts or something, but nope, just 2. So how come he likes her better? No idea at all, somehow her personality must match his a little better.
But it's fun and really works rather well - now I have a female sim who is crazy about her husband and a male who just hasn't quite found real true love with his wife. He likes her - the marriage works well, but... not all the sparks there could be. He'll probably have a midlife crisis, lose her and then end up sorry as friendship and shared experiences are important, but he doesn't realize it in the quest for the absolute dynamic

It'll also be interesting to see what that does to his son - said son being the whole point of creating them in the first place. However it comes out, I'm having a great time.