whats the point of leveling anyway? its either campaign and pvp right?
is there a co-op walking around in open world and fight bossess kinda thing? like borderland? or division?
Plains of Eidolon is the open world map. there are boss there. not sure do you need to accept quest to be able to find it. There are more boss in the other planets.
There are 2 type of level. One is your account level which is call Mastery rank, this one determine if you can use what weapon as weapon have mastery rank level limit. Mastery rank also determine some other stuff like how many trade you can trade per day and other stuff.
Second kind of level is your weapon, warframe and pet level. Higher level mean you can have more points to use more mods to power up your weapon or warframe or pet. Max level for all this is lv30. You can use orokin catalyst/etc to double the point.