Welcome to the site.
Does this site show preference for any particular form of Buddhism?
I hope that, theoretically, the site has no preference: there are at several meta-topics in which we say that we hope to invite questions about (and welcome people from) any or all forms of Buddhism.
Theoretically, also, unless otherwise specified the answers have no preference. So for example if you ask a question then you might get an answer from a Theravada or a Vajrayana perspective (or, perhaps even better, several answers from several perspectives).
One exception is if a question specifically asks for a specific perspective. So for example if you ask a question, and use a tag like theravada, then that question should only get a specific form of answer.
To give just one example, I can't find a single question referring to Nichiren Buddhism.
Yes: there are only a half a dozen questions tagged nichiren or sgi, so that's basically true.
I hope that's not because "the site" shows preference, but instead simply reflects the demographics or history of the site's users.
Even if a form isn't frequently-asked-about, a question about it might still get a good answer.
And if someone asks new questions, then possibly they might find some new answers.
Is there in fact a particular type of Buddhism that prevails on this site?
Possibly so; but among the site's 'regular users', who often answer questions, are various people who could answer from various perspectives including Theravada, Zen, Tibetan Buddhism, secular Buddhism, etc.
If so does it have a name so that I can find out more about it?
I think you can name but don't necessarily need to name a specific form of Buddhism, to ask a question on this site.
If not, what are the main teachings that people are discussing here?
I suppose, broadly, the teachings which people discuss include theoretical doctrine (for example the "teachings" such as are found in the Pali suttas and elsewhere); and about the practice of meditation.
Many of the questions appear to my untutored eyes to be referring to a default Buddhism that most people here adhere to.
Well, yes you may be right, but I think that's not quite so.
Instead I think that people often refer to their own understanding (whatever it is) when they answer questions; and that people's experience is fairly diverse.
Also people refer to other people's understanding, for example by including a reference with their answer, for further reading or so that you can see what the answer is based on.