This site is meant to replace RubyForge's gem hosting service. Replacing RubyForge as a whole is out of scope for this project. It has plenty of features (source code hosting, mailing lists, etc) that Gemcutter will never have. The real benefit here is a limited scope of features that can be focused more heavily on, and it's a full scale community effort that we can improve as time goes on.
There should be plenty of information on this Tender site to help out out. There's also been some great tutorials on using gemcutter, including a Railscast and several blog posts, some of the most notable by Adam Wiggins and Pat Allan. If you have a new problem, please feel free to open up a new Issue.
Through one command-
No. For new gems, once you push them you automatically gain control over it. For existing gems, you need permission to push to them. This permission can be granted by the gem owner, or if the gem was brought over from RubyForge you can use the migration process.
All of the RubyForge accounts have been migrated over to Gemcutter. If you need your accounts merged or you still can't push your gems, please open up a new Issue and we'll sort it out.
No. In fact it will make their lives better: it will be easier to get to project pages with clearer information and the same data they're used to.
Fork away on GitHub and make your changes! RubyGems.org is run on a 'commit bit' policy, so once you've submitted a patch that is accepted, you can gain access to the main repository if you'd like.