I’ve ran a lot of one-shots over the years. Some of them I still remember with great glee, but most of them have faded largely from memory.
One-shot RPGs are difficult to do well. I’d argue that they’re significantly more difficult than a typical campaign session. In a multi-session campaign the GM has longer to get things right, and there is more of a feedback loop for improvement. If the first session seems off, the GM an repeatedly readjust the game to make it jive with the players well.
In the one-shot, on the other hand, the GM is limited to small off-the-cuff changes, which means it’s on the GM’s shoulders to get it mostly right the first time. This can be very exacting, as it’s easiest to get it right the first time if the GM is working with known elements - players that are known well, a known system, a known setting, etc.
To make matters more complicated, in my experience one-shots are often used as a mechanism to test out new systems, new styles or play or engage with new players. While I understand the reasoning behind this (it’s a low-commitment way to try new stuff out), it’s also exactly at cross-purposes with trying to limit unknown elements in a one-shot.
In addition to limited room for feedback. one-shots are also obviously limited in time. They leave only so much room for players to digest a new character, come to grips with how the character works mechanically, get into a headspace to play the character and learn any new setting or system elements involved in the plot.
Despite their limits, however, one-shots have things going for them that campaign sessions do not. For one, the very fact that they are limited allows them to have a greater variety of character dynamics than is easily captured in a campaign. The player characters in a one-shot need not ever be on speaking terms again after the session is over. They can argue, they can go their separate ways, they can try to murder each other. So long as the plot is able to keep them together long enough to conclude the events of the session, it’s all good. In a campaign many of these dynamics would be problematic, obviously. If the PCs try to murder each other in session 1, good luck on having a smooth session 2, 3 or 4.
Because of their greater flexibility in character dynamics, one-shots also lend themselves to a greater variety of potential plots. This can be a nice change of pace. Some of the most memorable one-shots that I’ve ran are memorable at least partially because their plot structure was different enough to stand out from the usual structures of all the campaign sessions I’ve ran over the years.
These strengths do run into the fundamental limitation on time in a one-shot, however. Character dynamics and plots may be new and exciting, but players still have only so much time to unravel them or portray them.
Because of this, it’s particularly useful to portray characters or plot elements in a way that’s quickly digestible. Recognizable archetypes and tropes can be the GM’s best friend here. It gives players something to quickly latch onto without the need for extensive background information, memorization or rethinking.
System-wise, more simple or familiar systems are also useful here. It’s just one less thing to learn and remember in that small window of time that is the one-shot. It also reduces the chances that players will misunderstand something.