Since many "reasons" for changing talk pages to message wall are simply replied ad infinitum by staff, and the article comment implementation actually makes it harder to follow conversations (funny, isn't it?), I'm listing some of the fallacies here:

Talk pages are hard - you have to click edit and then use wikitext
Considering that this is a wiki, do we even want people that can't be bothered to properly use this core functionality to join conversations? - these people most likely won't be contributors, anyway. Also, keep in mind that there isn't much wikitext to learn for a simple request on a talk page, and there's something like a RTE to help with that.
Talk pages are hard - you have to make sure to sign your posts
Actually, being used to wiki communication for years now, I sign posts unconsciously. For users that keep forgetting that, there would have been a much simpler solution than a whole talk page redesign. For example, any edit of a *_Talk: namespace page could be parsed for a signature wikicode - and if such isn't found, it could either be added automatically, or the user warned.
Talk pages are hard - you have to get the indentation right
This is not difficult for users who are used to the indentation scheme of a community, and it can be fixed by others while writing a reply in all other cases. New users are more likely to start a discussion than to enter a discussion that already has a good amount of indented content, making this even less of a problem. On top of that - message wall's solution to this problem is to simply take away indentation, making conversations less structured, instead of offering some auto-indentation feature. If a community wants unstructured conversation, they could just get rid of any indentation schemes and instantly have "easier talk pages".
This can't be made optional, we want consistency
Consistency with what? Feature consistency across all Wikia - in which case, why are other features optional, like "article comments", or "badges", or "blogs", ...? Or do you plan to remove these inconsistencies in the future and force all communities to make use of all features? Or is it "consistency of the conversation experience" on each single wiki - in which case, what about the Talk: pages of all other namespaces except User:? Do you plan to change these to message walls, too?
Message walls are much easier to use
No, they aren't! It may be easier to start a message wall conversation than to start a talk page conversation (although not by much), but that is just one use case among many. A different use case is discussion management - spam and vandalism needs to be removed, redlinks (either after a page move or because a suggested page will not be created on a wiki) need to be changed, a longer discussion might need to be split into subsections. All of this is currently possible for any user, but will be restricted to a selected few (page owner + admins) on message wall. A third use case is a discussion move - just because a discussion is started on a user talk page doesn't mean it necessarily stays there forever. Policy discussions might be moved to Project_talk:, conversation about content to an article talk page, helpful advice to Help_talk: etc.. Again, all of this is easily possible now, with all discussion pages working alike - but won't be possible in the future, unless all other talk pages are changed into message walls as well. Finally, the most common use case of a discussion page is not to start a discussion, but to continue one. Even that isn't really easier or better with message walls, because of its restricted layout. There's no indentation making it easier to follow a discussion with many participants; every comment is confined to its tiny speech bubble, making it hard to use the discussion to suggest article changes (like layout/formatting changes); there's no editing interface allowing users to insert special characters, templates, etc.; there's not even a proper preview functionality to get things right the first time. Bottom line: message walls are not easier to use than talk pages.
Message wall allows features that would be impossible with talk pages
Untrue - at least if you allow for a modest amount of changes to talk page structures. About the only thing that is not possible with talk pages as they are is the "notification per thread" feature. This, however, could easily be implemented by changing talk pages so that a subpage is created for each new discussion, and then adding a forum-like "main page" (out of the box example: [1]; Note: a different suggestion, making use of the automated edit summary of a section edit, has been added as a comment below). Every other feature that has been brought up so far could either be added to current talk page functionality (like "auto-signatures" as described above), or isn't that big a feature in the first place.

...more to follow