I don't think that something being (more) mathematically rigorous explains all of what we see. Physicists at one time used to study fluid dynamics. Rayleigh, Kelvin, Stokes, Heisenberg, etc., all have published in the field. You can do quite a lot mathematically in fluids, and I have felt like part of some inner circle because of what I know about fluid dynamics.
Now the field has been basically displaced by quantum mechanics, and it's usually not considered part of "physics" in some sense, and is less popular than I think you might expect if a subject being amenable to mathematical treatment is attractive to some folks. Physicists are generally taught only the most basic concepts in the field. My impression is that the majority of physics undergrads couldn't identify the Navier-Stokes equations, which are the most basic equations for the movement of a fluid.
It could also be that fluids have obvious practical applications (aerodynamics, energy, etc.) and this makes the subject distasteful to pedants. That's just speculation, however. I'm really not sure why fields like physics, etc., are so attractive to some people, though I think you've identified parts of it.
You do make a good point about the sense of completion being different in engineering vs. social science. I suppose the closest you could get in social science is developing some successful self-help book or changing public policy in a good way, but I think these are much harder than building things.
I think there's also definitely a prestige/coolness factor which isn't correlated with difficulty, applicability, or usefulness of the field.
Quantum mechanics is esoteric and alien and weird and COOL and saying you understand it whilst sliding your glasses down your nose makes you into Supergeek. Saying "I understand how wet stuff splashes" is not really so... high status. It's the same thing that makes astrophysics higher status than microbiology even though the latter is probably more useful and saves more lives / helps more people - rockets sp...
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site!
Once a post gets over 500 comments, the site stops showing them all by default. If this post has 500 comments and you have 20 karma, please do start the next welcome post; a new post is a good perennial way to encourage newcomers and lurkers to introduce themselves. (Step-by-step, foolproof instructions here; takes <180seconds.)
If there's anything I should add or update on this post (especially broken links), please send me a private message—I may not notice a comment on the post.
Finally, a big thank you to everyone that helped write this post via its predecessors!