I've seen a lot of comment on, and some critique of desirable difficulties recently. They've come from a reasonably wide range of sources from across the education spectrum, but one thing a lot of them seem to have in common is an apparent misunderstanding of the word "desirable".
Desirable - "wished for as being [...] useful". The bold is deliberate. Useful. Useful difficulties. Difficulties what are useful. [Kuzco's poison]
because that's not useful, therefore it's not desirable.
Some of the suggestions for "things that might be desirable difficulties" would seemingly eliminate the "external support" bit while simultaneously adding to the "task quantity" bit. Which is.. not necessarily ideal. Ask yourself - in what way is this thing useful? Of course, as a student becomes more adept at a topic, you can remove some (all?) of the external supports, but that's because their prior knowledge basically takes over. They don't need the external supports so much anymore. What that doesn't mean is start printing your resources in Swahili. [Unless you're teaching Swahili, in which case, go right ahead].
I've seen people suggest using a tricky-to-read font makes things harder and therefore is a desirable difficulty. Is that useful? In some situations maybe, but generally? No.
Tasks should be challenging, they should stretch students to use what they know. They shouldn't be stupidly hard just for the sake of it. That isn't useful - it isn't desirable. Explanations shouldn't be tricky to understand because that's not useful, that's not desirable. You don't want your students to be sat there, trying to follow your explanation like:
Cracking out Adam Boxer's challenge equation here:
It is perfectly fine for students to be a bit stuck, as long as they actually have the tools readily to hand to get themselves unstuck. If they don't, then you've not created desirable difficulties, you've just created difficulties.
In terms of content and within a lesson*, how do you know if a difficulty is useful, if it's desirable? Well, what benefit does the student get from overcoming it? Do they have to bring together a bunch of threads from throughout the topic? Probably useful. Do they have to decode the question by doing a series of unrelated calculations [looking at you escape room tasks]? Probably not useful. Digging up the memory of content covered long ago in order to apply it to a new context? If they'll need that knowledge again in the future, almost certainly useful. Gleaning information from a long and rambling explanation, twinned with a slide covered in dense text? Not useful.
As usual, it pretty much comes back to "what are the students thinking about?" If they're thinking about the content, you're probably on safe ground. If they have to think about something else, probably best avoided.
*"Desirable difficulties" covers a whole lot more than I've talked about here - spaced practice, interleaving, conditions of practice - but I wanted to focus on this aspect rather than the whole spread because this is the bit that I've seen getting bandied about most recently. For more this is an easy introduction.