To begin with, short films in general, seem to be very low key. The fact that a short film does therefore need to be short, means that something expected from a feature length film, such as character names, is barely focused upon. They normally appear in the credits, if at all! This may of course be down to the fact that short films rarely have 'all star' casts, however even if they did, that wouldn't really change or improve anything. Also, in a lot of cases there tends to be one main protagonist, and in fact this protagonist may be the only character included in the duration. Moreover, this may also be due to the fact that short films tend to be very low budget, although this is because short films tend to focus on breaking boundaries through different aspects such as setting and story, rather than breaking banks on CGI or lavish studios.
From my knowledge, short films have also only recently had to be certified, with the introduction of distributers like iTunes. However, even with that being introduced, short films makers wouldn't really have to worry about limiting themselves in order to fit in a bracket, as they are distributed differently to films or film trailers. Short films are predominantly made through Indie film companies, who use more direct and cheaper methods to target their audience, using sites such as Youtube, Vimeo, Future Shots or social networking sites. This way they can simply upload their product and broadcast it to millions through the internationally accessible medium of the web. Short films can also be screened at film festivals and this can rapidly boost popularity.
In my opinion (and this is what I've gathered from every short film that I've watched), the main convention found within the art of the short film is originality. In comparison to feature length films, they seem to present themselves with ample creativity and steer away from what's seen as traditional. Short films are known to experiment with everything from camera shots, angles and framing to the music or soundtrack. More mainstream short films will share similarities to feature lengths in these areas although even they seem to have their own take on them. As far as narratives are concerned, these small scale productions never underestimate the viewer, and are incredibly enigmatic. Furthermore, they'll often end on some sort of cliffhanger though this may be with the incentive to make the film feature length.
I find that due to the low budgets that come with small productions, they tend to use flair and creativity to gather audience instead of gift wrapped glossiness and a list casts. It comes to across to me overall that short films are what they are down to the fact that they're done more for enjoyment than profit.
