Having a block qoute markup is a good idea and at least in Germany one important criteria for an accessible site. Besides a block qoute markup it would be also nice to have the inline version <q> </q> (see Inline Quote Proposal).

Note: blockqoute mustn't be used for indentation.

For accessibility it is always a good idea to use semantic markup (<strong>) instead of normal markup (<b>). Code elements could be marked with <code> and so on.

Here in Germany also these things must be met:

• easy ways to mark a certain passage/word with a "lang" attribute (in HTML), so that screen readers can adapt the pronunciation accordingly
• <abbr> and <acronym> are also very important to have. However this is not only markup I guess but calls for a deeper integration in the wiki engine (if you think of automatically resolving of <abbr> for example)