Y a-t-il des différences (sens, nuance, registre, conjugaison, etc.) parmi les propositions ci-après ?
Que se passe t-il ?
Qu'est-ce qui se passe ?
Qu'est-ce qu'il y a ?
Qu'est-ce qui arrive ?
"Que se passe t-il ?" and "Qu'est-ce qui se passe ?" are semantically entirely equivalent, the difference is only one of syntax and register: the first possibility is more formal but not so much, I would think, that for fear of sounding too literary one should forbid necessarily himself/herself the use of it in his/her speech . They both are a question about a more or less unusual situation of enough import and that inspires feelings of insecurity, danger, etc. They can be translated by "What's the matter" as this form connotes similarly those feelings.
"Qu'est-ce qu'il y a ?" et "Qu'y a-t-il?" (syntactic counterpart of second possibility above) are used nearly in the same way as the preceding two forms but about situations of lesser import, although the notions are quite relative. They can be translated similarly by "What's the matter?". Other possibility: Qu'est ce qu'il y a?/ Qu'y a-t-il? (spoken French: Qu'est-ce qu'y-a?) asked when nothing is expected to be happening, however, usually after a complete formulation of the question ("Qu'est ce qu'il y a [dans ta poche / au menu / de nouveau / derrière la porte /là-dedans ... ] ?" meaning "what is [there] ...") as a repetition of the question, but in abreviated form.
"Qu'est-ce qui arrive ?" is apparently not used as such: it is not complete and one will find rather "Qu'est-ce qui arrive à Jean? il ne fait plus ses devoirs, ses notes baissent!" or, for another example, "Qu'est-ce qui arrive à Jean? Il a tout d'un coup les meilleures notes de sa classe, je n'en crois pas mes yeux.". This form will however be found commonly in the tense "passé composé": "Qu'est-il arrivé?", "Qu'est-ce qui est arrivé?", and then the context of occurence is more often than not as for "Qu'est-ce qui se passe?" except for the change in time (past); the translation is commonly "What happened?". It seems fairly clear that "What's happening?" should not be translated by this form, albeit it is tempting and so apparently appropriate. There does not exit in French a true equivalent of the Anglo-Saxon "What's happening?"; a possibility sufficiently near is "Quoi de neuf?" or much less exact in the ways of satisfying to plain semantical correspondence, "Comment ça va?". "Y a-t-il du nouveau?", "Il y a du nouveau?" (somewhat informal), "Du nouveau?" are forms that correspond also to "What's happening?" but they are not proper in the context of people greeting one another unless there exists the common consciousness among the people in question of a particular situation that is important to them and that one person is inquiring about by means of those forms, and then their import as means of expressing greetings does not count for much: they are rather factual questions. One must be aware that there exists some variants of at least some of the forms advanced that are acceptable equivalents.