One of the things that was always stressed by one of my first editors (Richard Starkings) was to remember that every comic is someone's first issue. Therefore the plot should be clear enough to be understood by new readers, and the characters defined enough for people to know what part they play in the story.
In addition, I'd also say that art-wise, it should be clear what the characters look like and where the story takes place, so there should be at least one full figure shot if possible and enough background info to let readers know the location, even if the strip is only a page or less in length. (Classic newspaper strips were a masterclass in this kind of thing.)
Unfortunately, this isn't always the case these days, leading to confusing stories, which can fail to engage the reader and lead to them abandoning the comic. We often (quite legitimately) blame High Street retailers and their treatment of comics for falling sales but sometimes we as creatives have to shoulder some of the blame too. It's all about being aware... mindful, to use the modern term... of a clarity of storytelling that appeals to everyone, not just to the regular readers.
I'm not advocating that comics should be overly simplistic, just easy to follow, like all the best comics have been throughout history. This is especially important for children's comics where new readers will be unfamilar with the characters. Dialogue should also be relevant to the plot, not veering off into irrelevant banter. Yes, character interaction can be fun but flippant exchanges between characters sometimes drown out the plot when they're overdone.
With a week, a month, or more between issues, there should also be enough info for everyone to catch up on the plot. Sometimes readers are just dropped into a continuing story with little or no resumé. People have busy lives and although the creative team will remember what happened in the previous issue, the average reader might have forgotten. Since some creators started writing for the inevitable book collection, episodic writing has been in danger of becoming a lost art.
Most importantly though, even with complete stories, it's essential that a reader can follow the story. Comics should be clearly told, exciting, funny, and engaging for everyone. I know comics evolve and styles change (they always have, for over a century) but the fundamentals that make comics work remain the same. We can learn a lot from comics of the past in terms of how they kept their readers up to speed on character and plot.
I hope that doesn't come across as preachy. I'm sure most people reading this will know it already anyway, but it's just something I wanted to put down. One of these days, if I ever get around to writing an autobiography / how to do comics/ history of comics book, I'll elaborate on it there.