- Categories,
- Flagging,
- Markdown composer,
- File uploading,
- Sharing, Likes, Invites,
- Bookmarks,
- Wiki,
- Footer of topics like «created, last reply»,
- Suggested topics,
- Pages: About, Guidelines, etc
In my opinion, they must be delivered as a plugins 
But with one core design theme. They may be preinstalled, but I need a couple of big red buttons «Disable this and this, and this, and this».