Thanks for the great feedback. I think this is where Discourse is trying to make a distinction in "last post". Is the user looking for the most recent post, as you suggest, or is the user looking for the most recent post that he/she hasn't read? Once you start reading a thread/topic, Discourse tracks the posts you've read and can return to the very last post that you read once you return. Clicking the last post time (for example, 12h) from the Forums homepage, alternatively, will take you to the very last post made 12 hours ago, regardless of your prior reading history. What's the best course here? Last post in the topic or the last post in the topic that you've read?
I think the answer is, "it depends". As a brand new user, it usually makes sense to go through an entire thread, start to finish. For existing users, users that were here before the switch over to Discourse, it probably doesn't make much sense to start at the beginning, as they've read much of existing threads they follow. So the last post might make the most sense. But this is also where the Unread listing on the Forums homepage comes in. Instead of needed to search out the last post of a thread you're interested in, simply click on Unread to get a listing of all of the topics/threads that you are watching/following/have contributed to (read more about Notifications under the Preferences section).
One thing to remember, Discourse doesn't know what's been read while we were on vBulletin, so it will have to learn your reading habits from scratch. You are a new user in the eyes of Discourse. However, once you create or visit a topic/thread, regardless of whether you contribute to it or not, or change your notification levels, Discourse will remember where you left off.
Hopefully that sheds some more light on "last post" discussion. Let me know your thoughts. Thanks!