I will keep this post brief, for reasons that will hopefully explain themselves. I like to think I keep up with things these days. I thought I knew all the texting shorthand and what all that OMG, BRB, WTF, SMH, STFU and so on all meant (and apparently, F is rarely a good word). Then I got hit with one that actually says a lot about our world today as well as reminding me about an important part of writing. It's kind of long - five characters - but packs a wallop. It's TL;DR and this is something a writer never wants to see.
In the simplest way, we as writers need to grab our reader's attention and hold onto them until we are done. Since we only have words to do this, we better get to work. We have certain liberties - if the reader is picking up our 400-page novel, we have a few pages to draw them into our world and convince them to read another page, another chapter, and eventually give them an experience satisfying enough to persuade them to read another of our novels. If we write short stories, we have to grab them faster. That first paragraph, that first sentence needs to be a winner. Readers locked in from that first sentence will be more willing to read a longer piece than those who do not have an initial attraction.
Of course, readers also have expectations, and those better be met within a certain number of pages or words. In the world of blogging and commenting, words should be at a premium. Informative or inspirational blog posts should keep in the area of 750-1000 words - more than that goes beyond the purpose of motivation. Persuasive essays can be longer, and as these become more researched and use more elaborate arguments, they become downright huge. However, they still need to preserve the reader's interest with strong writing. Persuasive but dry papers might serve a purpose, but they lose their effectiveness as they get bigger.
When you write stories, you hopefully won't get hit with the TL;DR tag because your audience is people interested in the literary adventure. However, your obligation is still the same: To make sure they don't feel think your story is "too long" as they are reading. Those 300 pages should just fly by in no time. Kind of like this post, because I have hit my limit and I promised to keep this brief so nobody gives me that tag.