As someone who used to write wills for a living, I think that you've missed something out from your list of topics. (That's probably not surprising because it's not one that you normally have to deal with!). I refer to giving instructions (or more accurately, requests) relating to the funeral. To some people, getting their funeral arrangements in order is just as important as deciding who gets their money! (I don't share that view but I know it to be true of some people).
I suggest mentioning that any directions in the will relating to the funeral are not actually binding upon whoever arranges the funeral. (It could be worth pointing out that the best way of getting the funeral required is to pay for it in advance - but even that's not guaranteed). You might also want to refer to the impracticalities of burials at sea or in the back garden. (I wrote loads of wills though for people who wanted to be buried in a cardboard coffin, both to save money and to 'get back to nature' as quickly as possible).
How about mentioning, as well, the advisability (or otherwise) of specifying who should care for the testator's (minor) children? Setting up trusts for those children (and how the income and capital can be used by the trustees) might also be relevant.
Perhaps you could also advise against naming a solicitor as an executor? That effectively means that the value of estate will automatically be reduced through legal charges. I would regard it as better to appoint friends or family members as executors, so that they could then decide for themselves whether they needed legal help when going through the probate process.
A common question on here is "How long does it take for a grant of probate to be obtained, and the estate distributed in accordance with the will?". Yes, I know it's one of those 'how long is a piece of string' questions but I think that your audience might appreciate some general guidance on the matter.
Perhaps mention that witnesses are witnessing the signature on the document, not the document itself. (i.e. there is no need for them to read a single word of the will but they must actually see the will signed. Signing a will and then taking it to a relative's house for them to 'witness' it, is definitely not a good idea!).
Lastly, and most importantly, tell your audience that even if their will is perfectly drafted (by a Peterborough based barrister perhaps?), thoroughly checked by the ghosts of John Mortimer and Sir Edward Marshall Hall, double-checked by Lord Chief Justice Lord Judge and Lord Justice Goldring, and witnessed by every single Member of the House Of Lords, it will carry no weight whatsoever if nobody can actually find it! (Some discussion of where to keep the will, and who to tell about its location, would seem to be in order!).
Chris