New users may find it difficult to browse through the alphabetical list of "subs", many of which contain little or no content. Navigation would be simpler with an index which groups the "subs" into major topic categories, and indicates how many posts are actually in each sub-category.
Setting guidelines to limit the creation of new "subs" might also be beneficial, unless a new sub-category is really needed. Otherwise, the site is likely to become too cluttered to be useful.
Since all of the subs are added to every user's list by default, it makes sorting by "popularity" useless. Even if one of these categories has no posts or comments, the index still shows that it has over 7000 "subscribers". For example: /s/askhistorians