Please no silly drop-down to switch between tabs. That means a extra click/tap every time.
Well if you're going to a sub-item that's under the same major-tab you're on, then yes it's an extra click. But if you're going to a sub-tab that's under another major tab, then it's the same number of clicks.
I've never seen a major website that pulls off a horizontal scrolling list of tabs. That may work for apps, but this is the website.
Is that 'Help' drop-down needed? What is in it?
The wiki, faq, change log, and about page. It's possible this stuff could be moved elsewhere. The about page is already linked from the footer. The FAQ needs to be somewhere pretty prominent though, as it helps reduce emails to me about frequently asked questions like "How do I make a map". It's also nice for the wiki to be easily accessible.
is the Community drop-down to switch between say Community and Settings and Singelplayer etc? Or is it to go to different community things? Like Forum, Mail etc.
Community is similar to the community tab that exists now -- forum, ladders, tournaments, etc.
If you're in flash mode, there will be a single-player and multi-player tabs like there are now. But in Unity mode those aren't necessary, and you'll just have the Play button.