Importance of missions?

How important are the missions? I realize that some of them need to be done to advance in the game. As for the others is their a real reason to do them? Ive heard that they open up new quests, is this true? If so is it true for all missions? If not where can I find a list of missions that I should actually do? Thanks for any help guys.

The missions progress the story line, they move you quickly from one area to the next and can be easier than trying to find your way through the Explorable Areas. I'm not sure what you mean by the ones you should actually do. Each mission rewards you 1000Experience and a skill point so if you decide that you don't need the skill points you only need to do a few mission to have access to the entire map. You need to do the Sanctum Cay mission to access the desert. You need to do Thunderhead Keep to get to the Fire Islands. It you want to access the Tomb of the Primeval Kings with your PvE character then you also need to do the 3 missions in the Desert and Ascend. There are a total of 25 missions in the game. The following is a list following the storyline:
Great Northern Wall
Fort Ranik
Ruins of Surmia
Nolani Academy
Borlis Pass
The Frost Gate
Gates of Kryta
D'Alessio Seaboard
Divinity Coast
The Wilds
Bloodstone Fen
Aurora Glade
Riverside Province
Sanctum Cay
The next 3 Missions can be done in any order. The only way to find them is by exploring the desert.
Dunes of Despair
Thirsty River
Elona's Reach
The previous 3 Missions must be completed before Ascension can be done.
Augury Rock (Ascension)
Dragon's Lair
Ice Caves of Sorrow
Iron Mines of Moladune
Thunderhead Keep
Ring of Fire
Abaddon's Mouth
Hell's Precipice

Sorry I don't think I was clear enough in my first post. Ive actually completed the game going on 3 times now. However other than the 1k exp / skill point and transportation do the missions actually unlock anything? I've been told that they unlock new quests, is this true? If so I was wondering if ALL the missions unlock new quests.

well, Skill Points are important, at least later on. But, that's it. You know, unless you're one of those crazy types that plays a game for the story. You know. Like me.

Ok but completeing the missions doesnt unlock new quests?

Some mission's do unlock quest's after you do them

Like for instance after you do the Great Northen Wall mission, Warmaster Tydus and Captain Arne in Ascalon city will have quest's for you to do. The only quest you can get from either of them before doing the GNW mission is the Royal Message (go deliver note to Ambassador Zain) & "Defend the Wall" quest's from Warmaster Tydus.

So some mission's do unlock quest's, but to be honest I don't know what quest's get unlocked by mission's and which don't.