Working under the assumption that the question implies those elements not intentionally meant to be funny, but funny to modern viewers, here are some elements found humorous nowadays:
- The words on the screen do not match the length of time that the actors appear to be talking.
- The use of framed explanation of dialogue or mimed action seems unnatural
- The melodrama of the acting, exaggerated facial expressions, body movements, accompanying music, and information screens is oddly humorous to viewers who are now used to talking films with method actors. Here is an example of one "intertitle" cards used that is very melodramatic:
That night, there occurred the first of a series of mysterious crimes. (written in shaky letters)
- The make-up on the male characters seems exaggerated.
- The use of stock characters, such as the Victorian stock villain of black handle-bar moustache, sinister looks, dark eyes, and swarthy complexion and the bumbling police, etc. strike the modern viewers as comical.
- The closing of a scene by the shrinking of a screen seems rather silly.
For me there are a couple elements of silent films that make them humorous:
- Dramatic actions - like one of the above answers mentioned, the work of Charlie Chaplin comes to mind as his black-and-white figure bumbles around, bumping and crashing his way through obstacles (instead of around) while paired with playful, catchy jingles.
- Forced attention - being a silent film, the audience members have to keep their eyes trained on the screen through the duration of the entire film to catch all the action. I think this helps increase focus on the film so that we, as the audience, actually catch all the funny parts. Now, I find that if I am just watching a movie at home, I'll play around in my phone at the same time and will miss out on some of the funny conversations or situations.
- Dramatic irony - since there are fewer conduits of information in a silent film (the actors can't insinuate with their voices or their words what is coming up next), we as audience members are often given glimpses into what the characters may not be aware of. Again using the example from a previous answer, we know that there is a trap waiting for Tom as he chases after Jerry. We have to be shown, or at least in some way expect, this trap because if we are completely left in the dark, it doesn't make sense - why is there a pan that came out of nowhere to smush Tom's face?
- On the flip side, there's something to be said for the completely unexpected too. Because the actors can't give anything away with their lines/voices, we can be just as surprised when a character does something extraordinarily unexpected.
for me personally i think its the overdramatic actions like usually in alot of tom and jerry or Charley Chaplin etc.. but the music also helps