<readmore>...</readmore> will serve in many cases.

Note to Argel: Details? Writeup Formatting Tips.

... of course, succinct, precise exposition is preferable to verbosity.

Belated Update (20081218): Clarification, since I seem to have given offense.

As a reader, I favor readmores because:

• When I find a node of sufficient interest to read attentively (which is frequently), I usually want to read the post in its entirety. readmores allow me to do so by reading it directly in a new tab. spoiler tags leave me no choice other than to highlight them, whether viewed in the main thread or directly. I'm not suggesting that readmores are "more efficient" for me (and YMMV, anyway), but rather that I find opening a new tab to see the entire post suits me better than having to highlight the spoiled information... especially when a post contains multiple spoiled sections.
• The document referenced above addresses readmores as the way to "elide (hide) large chunks of a post, such as long sections of code" whereas Perl Monks Approved HTML tags explains the spoiler tag this way: "It is used for presenting "spoilers" to questions and will be rendered differently according to the users preferences, with the general idea being that a little more work is required to see the content than just viewing a node." As I understand that statement, the rationale is similar to that behind the practice of putting the answers to exercises in the Llama or, in newspapers, Soduku and crossword puzzles solutions on a different page than the puzzle itself... i.e., to reduce the likelihood that the reader will inadvertently see something that will "spoil" a challenge.
• spoiler tags (as someone else noted earlier) don't save me any screen space ... and IME, those folks who ramble/fail to make their point clearly/etc are also unlikely to take the trouble to format appropriately.

Yes, some folks ramble and wander from their topic with annoying verbosity -- but I can't think of a time when I've had that reaction to what you have written.