What? Did I play a different beta?
Destiny has 4-5 maps. They are instanced from each other. Further, when on a mission, even though you are in the "open" section" you get instanced when you go into the areas where you get the no respawn message. If you are behind someone when they enter a mission, they just vanish from your sight and enter their own little world. Most mission boss segments are also locked off in their own instances that only open when you are on the actual mission. IE: you can run up and kill the walking tank boss (pointless as he drops no loot), but the door behind him to the eye boss only opens when on the mission for it.
MP maps are the same as the single player maps, only instanced to specific locations on those maps. PvE and PvP are strictly separated.
Gun variety is small, and most ended up with the same guns. Where the challenge is I don't know either. Autoaim makes it so you can headshot most things, and while I can't speak for the so called difficult raid they will have at release, no mission in the beta was remotely difficult, even on hard. Not unless you look for the challenge in terms of playing higher level areas at low levels. If you are equal level to the area you try, it is not really difficult at all. The AI is incredibly easy to exploit as well.
IE: Majority of missions feature waves of mobs as the "difficult" part. They all tend to happen when you activate something. Mobs are programmed to path to that spot. You run away from that spot before the wave hits, and you can pick mobs off very easily because while they will path into that area, and follow you around if you are close to the activation spot, they won't really know how to hand you being behind them or to the side. You can then just pick them off at your leisure with the aggro follow range turning into World of Warcraft instead of what seems like EQ. Hell you can even find the sweet spot of aggro range, shoot them back off two steps, they turn to walk away, shoot them in the back, they turn towards you, run, then stop and spaz out because you are outside of their normal roam range of that encounter.