Here are some of the module that, pretty sure, should be included already:

• Answers, and the modules it depends on, i.e.: Rules, Userpoints, ... (what else?)
• Custom Rules, to implement business rules such as:
  • in this interesting answer about some wordlist validation (todo: maybe add it as an answers sub-module???).
  • ...
• Goals, for badge, achievements, etc. (note: have a look at the various SE badges also ...).
• Radioactivity, to provide a field type to be used as a hotness metric for entities (nodes, users, ...) and more, Refer to its community docu and/or this article for more details.
• Charts, for charting.
• Forena, for reporting.
• Group, to implement communities.
• Visitors, for site analytics.
• DrupalChat, (or Chatroom ? ) to allow chatting between authenticated users
• User Referral, to capture referrals.
• Membership Entity, to implement (paid) memberships.
• SimpleAds, to implement graphical or textual advertisements as configurable blocks.
• Rules block visibility, to configure block visibility using Rules Components.