I've only been using this beta for less than a day, but I've noticed some great changes. I think it's great that you have decided to update/upgrade the old forums into something new and different and I'm sure the new website style will be a huge hit.
New simpler looking buttons
More thematically linked colours on buttons
The search bar looks amazing
Everything generally is more aesthetically pleasing
I really like the 'go to last post' button when looking at latest posts in sub forum categories.
The about me page is awesomely fabulous
You can choose whether to display the 'Last edited by Jassycats: A moment ago' on an edited post
When you put someone's username in a post, it recognises it and puts a link to their profile (so cool!)
That little green dot is helpful to know if someone is online (without having to visit their profile)
When you hover over the +1 button, it says 'Like this post'
When you hover over the +1 button on a +1'd post it says 'You have thanked [username] for this post'
Pacman thing during loading when posting or editing a post
Faint shading on the 'Minecraft Forum' logo
Embedded videos look super cool
Do we really need to know how many people are viewing a page? (nope.)
I think the chests looked better to represent the different forum categories
I'm not a huge fan of those green blocks with speech bubbles on them (I liked the lava/water buckets)
Perhaps change the blue-grey headings on the homepage to a different colour (the ones that say 'Forums, Latest Post, Threads, Posts')
This was an issue on the old forum too but, I'll say it here too: some people signatures a too tall to see under their post; maybe add a mini scroll bar (I've seen this on other forums and it works well).
That's it for now, I'll notify you of anything else I might notice in the future. Hope this helps