An important point: Except in some relatively minor respects such as slang vocabulary, hackers don't get to be the way they are by imitating each other. Rather, it seems to be the case that the combination of personality traits that makes a hacker so conditions one's outlook on life that one tends to end up being like other hackers whether one wants to or not (much as bizarrely detailed similarities in behavior and preferences are found in genetic twins raised separately).
A substantial minority prefers `outdoorsy' clothing --- hiking boots ("in case a mountain should suddenly spring up in the machine room", as one famous parody put it), khakis, lumberjack or chamois shirts, and the like.
Very few actually fit the "National Lampoon" Nerd stereotype, though it lingers on at MIT and may have been more common before 1975. These days, backpacks are more common than briefcases, and the hacker `look' is more whole-earth than whole-polyester.
Hackers dress for comfort, function, and minimal maintenance hassles rather than for appearance (some, perhaps unfortunately, take this to extremes and neglect personal hygiene). They have a very low tolerance of suits and other `business' attire; in fact, it is not uncommon for hackers to quit a job rather than conform to a dress code.
Female hackers almost never wear visible makeup, and many use none at all.
(Role-playing games such as Dungeons and Dragons used to be extremely popular among hackers but they lost a bit of their luster as they moved into the mainstream and became heavily commercialized.) Logic puzzles. Ham radio. Other interests that seem to correlate less strongly but positively with hackerdom include linguistics and theater teching.
Further, hackers avoid most team sports like the plague (volleyball is a notable exception, perhaps because it's non-contact and relatively friendly). Hacker sports are almost always primarily self-competitive ones involving concentration, stamina, and micromotor skills: martial arts, bicycling, auto racing, kite flying, hiking, rock climbing, aviation, target-shooting, sailing, caving, juggling, skiing, skating (ice and roller). Hackers' delight in techno-toys also tends to draw them towards hobbies with nifty complicated equipment that they can tinker with.
For those all-night hacks, pizza and microwaved burritos are big. Interestingly, though the mainstream culture has tended to think of hackers as incorrigible junk-food junkies, many have at least mildly health-foodist attitudes and are fairly discriminating about what they eat. This may be generational; anecdotal evidence suggests that the stereotype was more on the mark 10--15 years ago.
In the U.S., hackerdom is predominantly Caucasian with strong minorities of Jews (East Coast) and Orientals (West Coast). The Jewish contingent has exerted a particularly pervasive cultural influence (see Food, above, and note that several common jargon terms are obviously mutated Yiddish).
The ethnic distribution of hackers is understood by them to be a function of which ethnic groups tend to seek and value education. Racial and ethnic prejudice is notably uncommon and tends to be met with freezing contempt.
When asked, hackers often ascribe their culture's gender- and color-blindness to a positive effect of text-only network channels, and this is doubtless a powerful influence. Also, the ties many hackers have to AI research and SF literature may have helped them to develop an idea of personhood that is inclusive rather than exclusive --- after all, if one's imagination readily grants full human rights to AI programs, robots, dolphins, and extraterrestrial aliens, mere color and gender can't seem very important any more.
Even hackers who identify with a religious affiliation tend to be relaxed about it, hostile to organized religion in general and all forms of religious bigotry in particular. Many enjoy `parody' religions such as Discordianism and the Church of the SubGenius.
Also, many hackers are influenced to varying degrees by Zen Buddhism or (less commonly) Taoism, and blend them easily with their `native' religions.
There is a definite strain of mystical, almost Gnostic sensibility that shows up even among those hackers not actively involved with neo-paganism, Discordianism, or Zen. Hacker folklore that pays homage to `wizards' and speaks of incantations and demons has too much psychological truthfulness about it to be entirely a joke.
Although high general intelligence is common among hackers, it is not the sine qua non one might expect. Another trait is probably even more important: the ability to mentally absorb, retain, and reference large amounts of `meaningless' detail, trusting to later experience to give it context and meaning. A person of merely average analytical intelligence who has this trait can become an effective hacker, but a creative genius who lacks it will swiftly find himself outdistanced by people who routinely upload the contents of thick reference manuals into their brains. [During the production of the first book version of this document, for example, I learned most of the rather complex typesetting language TeX over about four working days, mainly by inhaling Knuth's 477-page manual. My editor's flabbergasted reaction to this genuinely surprised me, because years of associating with hackers have conditioned me to consider such performances routine and to be expected. --- ESR]
Contrary to stereotype, hackers are not usually intellectually narrow; they tend to be interested in any subject that can provide mental stimulation, and can often discourse knowledgeably and even interestingly on any number of obscure subjects --- if you can get them to talk at all, as opposed to, say, going back to their hacking.
It is noticeable (and contrary to many outsiders' expectations) that the better a hacker is at hacking, the more likely he or she is to have outside interests at which he or she is more than merely competent.
Hackers are `control freaks' in a way that has nothing to do with the usual coercive or authoritarian connotations of the term. In the same way that children delight in making model trains go forward and back by moving a switch, hackers love making complicated things like computers do nifty stuff for them. But it has to be their nifty stuff. They don't like tedium, nondeterminism, or most of the fussy, boring, ill-defined little tasks that go with maintaining a normal existence. Accordingly, they tend to be careful and orderly in their intellectual lives and chaotic elsewhere. Their code will be beautiful, even if their desks are buried in 3 feet of crap.
Hackers are generally only very weakly motivated by conventional rewards such as social approval or money. They tend to be attracted by challenges and excited by interesting toys, and to judge the interest of work or other activities in terms of the challenges offered and the toys they get to play with.
In terms of Myers-Briggs and equivalent psychometric systems, hackerdom appears to concentrate the relatively rare INTJ and INTP types; that is, introverted, intuitive, and thinker types (as opposed to the extroverted-sensate personalities that predominate in the mainstream culture). ENT[JP] types are also concentrated among hackers but are in a minority.
As cynical as hackers sometimes wax about the amount of idiocy in the world, they tend by reflex to assume that everyone is as rational, `cool', and imaginative as they consider themselves. This bias often contributes to weakness in communication skills. Hackers tend to be especially poor at confrontation and negotiation.
Because of their passionate embrace of (what they consider to be) the Right Thing, hackers can be unfortunately intolerant and bigoted on technical issues, in marked contrast to their general spirit of camaraderie and tolerance of alternative viewpoints otherwise. Old-time ITS partisans look down on the ever-growing hordes of UNIX hackers; UNIX aficionados despise VMS and MS-DOS; and hackers who are used to conventional command-line user interfaces loudly loathe mouse-and-menu based systems such as the Macintosh. Hackers who don't indulge in USENET consider it a huge waste of time and bandwidth; fans of old adventure games such as ADVENT and Zork consider MUDs to be glorified chat systems devoid of atmosphere or interesting puzzles; hackers who are willing to devote endless hours to USENET or MUDs consider IRC to be a real waste of time; IRCies think MUDs might be okay if there weren't all those silly puzzles in the way. And, of course, there are the perennial holy wars --- EMACS vs. vi, big-endian vs. little-endian, RISC vs. CISC, etc., etc., etc. As in society at large, the intensity and duration of these debates is usually inversely proportional to the number of objective, factual arguments available to buttress any position.
As a result of all the above traits, many hackers have difficulty maintaining stable relationships. At worst, they can produce the classic computer geek: withdrawn, relationally incompetent, sexually frustrated, and desperately unhappy when not submerged in his or her craft. Fortunately, this extreme is far less common than mainstream folklore paints it --- but almost all hackers will recognize something of themselves in the unflattering paragraphs above.
Hackers are often monumentally disorganized and sloppy about dealing with the physical world. Bills don't get paid on time, clutter piles up to incredible heights in homes and offices, and minor maintenance tasks get deferred indefinitely.
The sort of person who uses phrases like `incompletely socialized' usually thinks hackers are. Hackers regard such people with contempt when they notice them at all.