So I finally did it, I stepped into the Arena yesterday night with my 4 shaman and another person player Druid or Paladin (same person).
They're all 18-20k hp, around 2.2 - 2.5k sp self buffed, although also high variations in terms of resilience (938-850-580-460).
Only two of them are fully pvp geared with all you can buy without doing arena (honor, emblems, wg gear,...).

I did a 9-18, which was positively surprising. I really expected to be totally destroyed. The big contrast here is that either I faced teams that totally destroyed me in a few seconds or teams that I totally owned pretty fast (well, at least 3-4 of them, turning the rest of the match to be some pursuit thing....).

I haven't read much of the new system, I have to, I don't understand it really well as far as I've seen it, but one thing that comes to my mind from the S4 which I played for a couple of months is that I find the matchmaking system to be friendlier to newcoming teams and the notion of not losing rating points (although a much slower climb) is quite nice at least from a psychological point of view.

With this little experience I still don't figure out much how to handle Dalaran and Orgrimmar Arenas since I never knew them, so I'd appreciate some feedback on how you do it.

We did much better when my friend played the paladin (7-11) which might be related to the fact that he can survive longer (better gear too).
I intend to, during this week, try also two other healers, a Priest and a Shaman, both friends of mine too and try to figure out what the best compo might be for me. I also have a huge learning curve ahead of me. AoE is less a problem in Arena than what I usually face in AV (because there's less people doing it ) but the group stuns are still my worst enemy.
Some teams I lost against earned 2 to 5 points and it was (naturally) impossible to down at least one of them.

I have a big problem with my display setup and I can't seem to find an optimized way of doing it. I run with two computers, two monitors (main on one, alts on the other) and everytime I lose my main it's really disturbing the drastic change of view as well as passing to a smaller (much) window.

I hope to keep updating this post sharing my experiences, since they're utterly basic so far and would also appreciate any kind of feedback from the community.

Cheers and have a great day!