1. Might not be in COMMON interest
2. Good idea. Should be done by core.
3. It might be good for SEO. It can be offered optionally at admin setting of q2a by core.
4. There is already "All Answers", "All Questions", and "All Activities" in the user page. But I guess you mean "All Answers" shows only title of question, not the answer content written by the user. Am I right ? In my website I have fixed it. I guess it should be fixed by core as well.
You can check at:
***It is in Turkmen language. "Entry" is label for questions, "Teswir" is for answers, "Jogap" for comments.
5. This seems to be the main issue that should be done by core. 123.jpg format is also good in terms of SEO and performance.
6. Favorite functionality can do the subscription tasks. See MyFavs-Gyzgyn plugin. It adds my favorite tags, my favorite categories widget. If you want to see all questions relating to your favorited tags (especially when fav tags are more than one) in one q_list, then this plugin does not do this. But it can be augmented. It is open-source at github.
7. Subscribers/Followers/Fans. This can be done by querying how many users has favorited the USER-A. With simple plugin it can be done. If it is implemented by core, it would be better as it might be in Common interest.
8. I guess this is available. You just need to set on admin side settings. Am I wrong?
9. There is a notification plugin available for this.
10. This can be done by plugin. If it is implemented by core, it would be better as it might be in Common interest. But it will kill some performance of website.
11. Like/Dislike in q2a is up/down vote. It can be just renamed. But functionality will be same.
12. There is just a plugin that sends email to all users, but I am not sure if it sends email to group of users. But it can be done by plugin.
13. Same comments as 12.