I have been running many different boards as admin trying to build communities. So I am approaching the great vanilla from a people's not just a technical perspective. Most of these suggestions are meant for later versions of vanilla - maybe as plugins (love that idea!!!).
Rating / Best Answer
Sometimes when you browse a forum, you want to 'find' the best answer to some topic. Here some form of rating or meta moderation function would be great. Metafilter has recently introduced some sort of 'best answer' function, Slashdot has meta moderation others use some sort of rating systems.
In the spirit of vailla I would prefer something simple, like a [+] button to mark an answer / posting as special.
There is also a twist: it should show (maybe as a small symbol / number) how many ratings the posting got from others - but also should keep YOUR personal rating as well - so you can find them later.
In connection with the last suggestion and the current bookmark system I have two ideas: let me bookmark single postings and give me a function in my users settings to edit/delete all my bookmarks. Just jumping back to the old discussions and 'unsubscribing' is a LOT of work once you have been on a board for some time. Maybe folders for bookmarks is too much to ask. Maybe something for vanilla 2.o?
Bookmarking people would add my next point:
Bookmark people: Friends & Enemies
Forums are often used like social places - so friendship lists or even enemy lists are a great help. Great social engineering tools. So why not bookmark people as well? You already have a 'negative' bookmark function: block user.
Metafilter allow user to link to each other. Slashdot offers all sorts of lists / social relationships.
Tribes, Clubs and Postings
I am sure many gaming boards and social forums would love a function so people could organize themselves into groups (tribes, clans, whatever you wanna call them). Sure people can use the custom fields to add some sort of 'clan' function - but it would be nicer if you could have a proper function for it: starting a clan, clan elders, members and applicants. More or less the same functions like the admin already offers - only for members.
In that context it would be nice if clan people could create postings only visible to their own team/clan. Very groovy!
Personal profiles should include a simple button so you can search / list all of a persons postings. The board already has the functionality, but a small button would simplify the access. Also a great social egineering tool - because it's easier to get to know other members if you could study what they write / behave.
Fields in Profile
I love the easy custom field options in the user profile. Would love to see the idea extended with fields the admin can create and which appear on EVERY profile. The admin should also be able to make some of them mandatory. Obviously you also want to be able to search those fields: like in a single community you can search for certain hobbies or someone who like the same TV show as you. Plastic.com did this very nicely (although a bit complex).
Thanks for reading.