Psychological theories of humor are a growing area of research. I attended a seminar on it for a day as an undergraduate myself, but dropped it before giving the instructor a chance to demonstrate that there was much detailed knowledge out here. Also, this was several years ago, so I'm sure the field has progressed since then.
The "funny because it's true" factor may explain why cleverness and originality are important. Myself and many others find stimuli funnier when we haven't encountered them before, and especially when they violate expectations. Novelty is an important factor in the experience of pleasure in general (Lyubomirsky, 2013), and this clearly applies in humor to some extent, in that jokes often "get old" eventually. At first hearing, a joke may offer more opportunity for new insights (depending on whether one "gets it" right away; this also explains why one generally won't laugh until one does get the joke), though it may continue to remain funny as one continues to process new insights gained from reconsideration of the joke's implications. I often find jokes less funny when they express misconceptions or depend on an ignorant perspective that I don't share (e.g., the suggestion that some apparent contradiction is absurd doesn't amuse me as much if I know the reason for the apparent contradiction already).
This isn't to say that something needs to be true or representative of reality in general to be funny though. Absurdist humor in particular relies on the violation of expectations in ways that seem coherent at least (i.e., it probably won't be funny if it's just too confusing or bizarre to make any kind of sense), but in ways that may seem particularly absurd (and probably novel too) due to violations of the laws or probabilities of physical or social reality. This principle suggests one reason tastes may vary in the sense of humor: individuals vary in their tolerance for expectation violations, and in their abilities to make sense out of them. Openness to experience relates to cultural preferences (e.g., traditional vs. avant-garde), level of educational attainment, and (negatively to) need for closure, which suggests a particular hypothesis of individual differences that may be worth testing: openness may relate positively to appreciation of absurdist humor, and negatively to appreciation of "blue-collar" comedy (at least when the latter appeals more to simple, traditional, or uneducated sensibilities, which it doesn't always).
The nervous laughter factor may increase laughter as a sort of coping mechanism for social discomfort. This may partially explain the popularity of humor on taboo topics such as political incorrectness (e.g., Politically Incorrect), demographic (e.g., gender, age, and ethnic) stereotypes, general immorality (e.g., "dark humor"), sex, and "toilet humor". By nature of being taboos, experience with these topics and exposure to a variety of opinions on them are commonly limited, so this factor may depend partly on the novelty factor.
However, nervous laughter does not involve novelty necessarily. Self-perceptive processes may produce the impression that something is funny because one is laughing, even when one is laughing for other reasons. Various social reasons may disincline people to express anxiety or discomfort directly (e.g., if a friend or comedian is obviously trying to amuse you with something distasteful), and forced laughter is a common and sometimes habituated way of hiding negative emotions through an opposite expression (a reaction formation) of levity and enjoyment. This reasoning appeals to a somewhat circular theory of humor (i.e., our laughter defines humor as that which makes us laugh), but people use circular logic in many ways (e.g., early behaviorism has been criticized as circular). Therefore, to whatever extent humor is a social construct by definition, theories of humor need not be less valid to the extent that they are circular and simply descriptive of thoughts, feelings, and behaviors that relate to laughter. Over time and repeated exposure, people may come to commonly regard a topic as humorous simply because it provokes laughter, even if that laughter is only a nervous reaction initially.
A particularly loose line of reasoning that could tie nervousness to laughter would apply this explanation to the laughter reaction to tickling, which is a particularly odd unconditioned response (that seems to occur in animals too). Again, just a bit of personal speculation based on evolutionary theory, FWIW: in mammals, ticklish areas are often vulnerable to physical injury, and therefore sensitive. These areas seem to generate an odd sort of nervous discomfort reaction and motivate defensive movements reflexively, maybe to naturally prioritize protection of these areas from injury. Natural selection may have engendered such a reflex to extended this reaction even to sleep or intimate contact with trusted others (which could cause injury during aggressive play, sex, or by betrayal). Experience of this defensive reflex in action during seemingly inappropriate circumstances may generate laughter through surprise (thus involving the novelty factor again) or through reaction formation to nervous discomfort, among other possible explanations. For support of this explanation, note that the laughter response to tickling can be suppressed intentionally when the experience of tactile discomfort remains.
Conformity may cause laughter and tickle the sense of humor in similar ways. Following the above reasoning somewhat, self-perceptive processes may kick in after one laughs to mimic others nearby. People conform to others' behavior for a wide variety of reasons, and in a wide variety of ways. This may explain why people sometimes enjoy stand-up comedy more when attending live performances than when watching them recorded from home, and why many television comedy shows use laugh tracks.
One explanation of laugh tracks would again involve novelty somewhat. Subtlety in humor is often an expression and consequence of cleverness and originality in the delivery of a message, as by implication rather than explicit expression (comedians often try to avoid "beating" their audiences "over the head" with their punchlines). Yet again, one must notice and (often, but not always) comprehend a stimulus to find it funny. Thus a laugh track serves to cue the audience to parse stimuli for humorous content, and may help compensate for individuals' varying abilities to detect subtle stimuli or their implications. Cuing appropriate responses to ambiguous stimuli and situations is a common function of conformity in general.
Conformity occurs even in the relative absence of ambiguity though (cf. the Asch conformity experiments). This suggests another explanation of laugh tracks: a pure conformity manipulation aimed at manipulating humor judgments through self-perception. This explanation may apply more broadly to the commonly experienced social phenomenon of "contagious" laughing fits in groups, which some special groups capitalize upon (e.g., Laughology, Laughercise). Yet again, laughter may result in response to the novelty and seeming absurdity of such events as well.
Furthermore, the social contagion of laughter and some instances of animal laughter suggest as an additional principle that laughter may arise as a simple behavioral manifestation of positive affect in general. Joy and exuberance may naturally result in laughter as an exaggerated form of grinning. In this sense, intensity of the stimulus becomes an important factor unto itself; consider how much harder one laughs at something especially hilarious. Next, consider the common emphasis on surprise as a comedic element, and the opposite effect of having to explain a joke. A sudden surprise may load all of the stimulus' information content into a briefer, more intense moment, thereby intensifying the emotional reaction. Conversely, spreading out that information over a long-winded explanation may spread out the emotional reaction so much that one no longer finds the joke funny for lack of emotional intensity, even though the same amount of information (and sometimes more) is ultimately conveyed.
Because positive affect is generally desirable, and laughter is generally enjoyable, people may force laughter somewhat simply to enjoy themselves more, and this need not occur consciously. This implicit motive may lower people's thresholds for laughter (and thus the perception of humorousness) in situations where positive affect is generated by other factors, and maybe also in situations where positive affect is lacking (e.g., in the midst of sadness, frustration, or anxiety). Consider as (somewhat circumstantial) evidence how popular media such as movies often mix comedy and tragedy to accentuate each.
Yet again, this effect may be partially mediated by the novelty factor if one is surprised at the depth of one's joy. The other social factors I've mentioned here might conceivably play mediating roles too. For instance, extreme joy might heighten emotional arousal past comfortable levels, leading to anxiety and nervous laughter; or one might force laughter to communicate a desire for levity or to manipulate one's social atmosphere through conformity-based mechanisms. This last point may apply especially to laughter as a direct coping mechanism for negative emotions: laughter may not only hide negative emotions, but may overwrite them somewhat with positive emotions.