I have to quote some parts of your post so it is easier to understand.
In the wiki (and wikipedia) it's said that manjaro is 100 % compatible with linux arch. Is it really true ? I mean the AUR seems to be the same as arch but not the default packages repository right ? About that, what is precisely in each repository ? Does the unstable is in fact the default arch packages or other packages made by the manjaro team ? This point is really important to me because arch is a quite big distribution and being fully compatible with it would allow users to get help and ressources from the arch community which is significantly bigger than the manjaro one. For example, if I get trouble with softwares (or hardware) can I ask for help in the arch forum or people won't want to help me because I have different packages ?
To answer this, you have to know how Manjaro works. We are using Archlinux binary packages from Arch-Stable Repositories.
Not all packages are used from Arch. 99% of Arch-Packages you can find in platform (core), extra, community and multilib.
Additional to this four Arch-Repos we have our own repos called basis, basis-multilib and addon.
In basis you find stuff linke our kernel, kernel-extramodules, filesystem, systemd and other modified packages mostly found in core on Archlinux. basis-multilib handles modified lib32-packages. In addon you find additional packages you might find in extra, community and AUR.
So you know now how our mix is set up. Now to our different layers: stable, testing, unstable.
Unstable, you find mostly daily synced packages from Arch-Stable. It depends how often I or other core-developers have time to sync from Archlinux. In this layer we adopt our latest changes and test them for 1-2 weeks before we move them to testing.
Testing, is designed for a wider user-base. Those packages are already tested by the core-team and signed of as "it-works".
Now our test-squad kicks in and tell us what we might have missed. All our install-media testbuilds get created from Manjaro-Testing. Those are tagged as X.Y.ZpreA. Our latest testbuild is 0.8.3pre3 and was released today.
If our test-squad send us their feedback and we found a solution for those issues and all is fixed for prime-time we move testing to our stable repos and announce that on our blog. If there are bigger changes even Unstable/Testing merges get announced on our blog to make live easier.
Stable, is what our normal Manjaro-Users have running. You get our stable repos by installing Manjaro with our official release.
Latest stable install-media is 0.8.2. So in the end Manjaro-Stable lacks on freshness about 3-4 weeks from Archlinux.
AUR-Packages are supported but sometimes don't work due outdated packages in our repos.
Again about packages, how many servers are available for software download/update ? It seems to me that fetching software using pacman is slower in manjaro than in arch. I have not looked into server ranking yet but I hope it is automatically done at install (if not you know what feature you can implement for the next release 
)
Speed is low since you all are on our UK-Mirror. As Manjaro is more popular now, you might want to change to a
faster mirror. It is well explained in our
wiki. Latest mirrorlist is available from
our git.
Then, I got two minors problem for now with manjaro. One is directly related to the distribution. Kalu said again and again that I have 10 unread news (probably from the rss feed) and it starts to be annoying ? Do I miss something or is there a way to tell him to not show that message again ?
Finally, I got some weird display bug in opera when using the right click menu. Any idea about that ?
It depends on the edition you have installed. On Cinnamon/Gnome edition you don't have the "mark as read"-button. How to get rid of that news notifications I
already answered. As for Opera I don't know.
