I Miss "Today's Active Posts"
That's okay - you can still see them, it is just in a different place in this new software. In the blue bar above the forums, there's a spot marked New Posts

I Miss "Top 25 Posters"
That's okay - you can still see them, it is just in a different place in this new software. When you view the Members List you can click on "Posts" to sort by who has the most posts, or by "User name" to see everyone listed alphabetically.

I Miss being able to see everyone's birthday that's coming up this week under "Calendar"
That's okay - you can still see them,just switch your Calendar View to Weekly View instead of Monthly View