Bad slide navigation on the web and understanding why it's bad
As usual, I'll start with my tweet:
If the online copy of your slide presentation is structured in 2D, not just 'go forward', please know that I just closed my browser window.
This is sort of opaque because of the 140 character limitation, so let me unpack it.
People put slide decks for their presentations online using various bits of technology. Most of the time how you navigate through those decks is strictly linear; you have 'next slide' and 'previous slide' in some form. But there's another somewhat popular form I run across every so often, where the navigation down in the bottom right corner offers you a left / right / up / down compass rose. Normally you go through the slide deck by moving right (forward), but some slides have more slides below them so you have to switch to going down to the end, then going right again.
These days, I close the browser window on those slide presentations. They're simply not worth the hassle of dealing with the navigation.
There are a number of reasons why this navigation is bad on the web (and probably in general) beyond the obvious. To start with, there's generally no warning cue on a slide itself that it's the top of an up/down stack of slides (and not all slides at the top level are). Instead I have to pay attention to the presence or absence of a little down arrow all the way over on the side of the display, well away from what I'm paying attention to. It is extremely easy to miss this cue and thus skip a whole series of slides. At best this gives me an extremely abbreviated version of the slide deck until I realize, back up, and try to find the stacks I missed.
This lack of cues combines terribly with the other attribute of slides, which is that good slides are very low density and thus will be read fast. When a slide has at most a sentence or two, I'm going to be spending only seconds a slide (I read fast) and the whole slide deck is often a stream of information. Except that it's a stream that I can't just go 'next next next' through, because I have to stop to figure out what I do next and keep track of whether I'm going right or down and so on. I'm pretty sure that on some 'sparse slides' presentations this would literally double the amount of time I spend per slide, and worse it interrupts the context of my reading; one moment I'm absorbing this slide, the next I'm switching contexts to figure out where to navigate to, then I'm back to absorbing the next slide, then etc. I get whiplash. It's not a very pleasant way to read something.
Multi-option HTML navigation works best when it is infrequent and clear. We all hate those articles that have been sliced up into multiple pages with only a couple of paragraphs per page, and it's for good reason; we want to read the information, not navigate from page to page to page. The more complicated and obscure you make the navigation, the worse it is. This sort of slide presentation is an extreme version of multi-page articles with less clear navigation than normal HTML links (which are themselves often obscured these days).
I don't think any of this is particularly novel or even non-obvious, and I sure hope that people doing web design are thinking about these information architecture issues. But people still keep designing what I think of as terribly broken web navigation experiences anyways, these slide decks being one of them. I could speculate about why, but all of the reasons are depressing.
(Yes, including that my tastes here are unusual, because if my tastes are unusual it means that I'm basically doomed to a lot of bad web experiences. Oh well, generally I don't really need to read those slide decks et al, so in a sense people are helpfully saving my time. There's an increasing number of links on Twitter that I don't even bother following because I know I won't be able to read them due to the site they're on.)
Sidebar: Where I suspect this design idea got started
Imagine a slide deck where there you've added some optional extra material at various spots. Depending on timing and audience interest, you could include some or all of this material or you could skip over it. This material logically 'hangs off' certain slides (in that between slide A and F there are optional slides C, D, and E 'hanging off' A).
This slide structure makes sense to represent in 2D for presentation purposes. Your main line of presentation (all the stuff that really has to be there) is along the top, then the optional pieces go below the various spots they hang off of. At any time you can move forward to the next main line slide, or start moving through the bit of extra material that's appropriate to the current context (ie, you go down a stack).
Then two (or three) things went wrong. First, the presentation focused structure was copied literally to the web for general viewing, when probably it should be shifted into linear form. Second, there were no prominent markers added for 'there is extra material below' (the presenter knows this already, but general readers don't). Finally, people took this 2D structure and put important material 'down' instead of restricting down to purely additional material. Now a reader has to navigate in 2D instead of 1D, and is doing so without cues that should really be there.