Hello,
New/ly returned player here, and have had similar concerns to a degree.
Playing GW is a bit different than other mmo/rpg games in several ways. You've come across several thus far
Torso and legs I believe are the most important to get coverage for (takes the most hits/dmg)
Here is a great resource for you (and me heh)
http://wiki.guildwars.com/wiki/Armor
As far as Explorable Area...I believe there are areas closer to what you are looking for farther along in Prophecies. I know that NightFall has larger areas as I was there yesterday checking things out heh. But I haven't reached them yet in the first game/chapter either. (lvl 11W/Mo)
http://wiki.guildwars.com/wiki/Explorable_area
Sure there's a list of common used abbreviations here in a thread or on the wiki/web, I'd check there for that...or search here
As far as the lvl cap at 20...at first the idea of that rubbed me the wrong way too. But I guess I've become used to the concept. For me new areas, quests and items are the most important thing, I believe you can still acquire points for Skills and different Titles and such, so there is a continued ranking of sorts long past the hitting 20 stage of the game.
I had a reply to some of my own questions that I read today that I think would suggest and help you form an idea of what a possible game play path might be, I gladly share that with you (hope the OP doesn't mind

):
*********************************
Nechronius's recommendations were to:
1. Buy all the expansions.
2. Get your main warrior to Lion's Arch so you can go to Nightfall.
3. From Nightfall the very first quest offered is to get several heroes.
4. You will also get to choose between a ranger and an elementalist. I recommend getting the elementalist.
5. From Kamadan, request a "ferry" to Consulate docks but don't pay more than 500gold for this.
6. Get the quest to pick up Olias as a hero (all for one and one for justice).
7. Use your new-found heroes to complete Prophecies.
8. Use your heroes to beat Factions.
9. Use your heroes to beat Nightfall.
10. Use your heroes to beat Eye of the North.
You will pick up a lot of heroes along the way (well over a dozen across nightfall and EotN) but general consensus in my guild is that the necromancer is your #1 draft pick, so to speak, which is why you will really want to get Olias as soon as possible. A properly set up minion master necro will make mincemeat out of the vast majority of the gaming areas in all campaigns and is a huge tactical advantage, not only with extra damage dealers but also the body blocking and extra damage soakers. And if an MM necro is not appropriate, a cursing necro with the Spiteful Spirit elite is also an awesome way to sweep aside your enemies.
I don't intend this to be a strategy guide, but having heroes will add a much greater depth to your gameplay experience if you are primarily doing the game with henches. Once you get Olias you can easily finish the campaigns in order with a single character, which is what I did with my warrior. It makes you feel like you're a visitor to a foreign land experiencing a little bit of culture shock. Plus the storyline flows together fairly well.
EotN is somewhat independent of factions/nightfall so you could probably do it first before the other two campaigns, however EotN doesn't feel like it has a really definite "end" like the other two campaigns do so I prefer to save it for last.
Definitely finish factions before you finish nightfall though, as the end of nightfall has a little bit of a spoiler about factions. however get Olias first.
****************************************
As I'm basically only concerned with PvE, this will work as a basic outline or plan for my journey through Tyria (the world of GW)
Perhaps it will help you as well
I want to say that the community here is great, very willing to help with advice and answers to questions a player might have.
I think with your own exploring and learning...and the resources found here and the GW wiki pages you'll have plenty to read/learn and study, not to mention "try" and experiment with
It's a fun game!
So Good Hunting & Regards