1) The explore part of the game is where the open world is or when you're on a mission and go get yourself sidetracked. It is far from a mission picker. When you go to a mission you don't actually have to go one linear direction either, there's no walls to block you, the whole world map is open to you.
2) The load times in the tower are because of the game having to instance your cryptarch, messages, vanguard ranks, inventory timers for the npcs, the inventory the npcs have, as well as load in all the players that may be there.
3) The weapons use different ammo, so yeah it's intended. You should (I hope you are buying them) bring the ammo packs from the weapon npc. Heavy ammo drops like water from a leaky faucet for me. I see it all the time, and generally you shouldn't be using up your heavy ammo that much anyhow.
4) The enemy AI doesn't react much if you're far away probably because they cannot tell what direction you are shooting from. If they can see you, they will attack you. I've had some enemies start attacking me from considerable distance. Across the map no, but pretty far away.
5) I've found some spinmetal plants that wouldn't allow me to pick them up. As if the server would not sync to me. I came back in a few minutes and it worked fine. Perhaps your chest was the same way.
I know the beta pretty much by heart now. I hope even the Earth area will be a lot bigger than what is in the beta, because otherwise this thing will be pretty small.
As for the ammo... primary weapons don't use same ammo and yet they do not reset? Also, I'm not saying a shotgun should shoot sniper rounds, but the game should remember how much ammo you have for a particular weapon type. Right now I can have full machine gun ammo, switch to a rocket launcher (get 0 rockets), switch back to machine gun and get 0 machine gun bullets... what happened to them? As for "shouldn't be using" - huh? It's fun running around dropping things with a machine gun, why should I not want to use it?
The sucky AI can be seen for example when fighting Thralls, especially in the last story mission's final fight. They just run around not really attacking me, as long as I do not cross a "magical line" - it's not even that far away from them... Also, the mobs in general seem to be hiding a lot, peeking behind cover and not really attacking. I can just run around melee'ing stuff as I see fit without much trouble.
And the load times? Please, loading a few values takes this long? Loading other things like textures? The console has 8GB of RAM, even if not all is used by the game, that still is a LOT. Starting the game - loading screen. Selecting destination? The ship "thinks a bit", does a reverse and heads towards the planet (takes a while...), then the screen goes black and you see an animation of the ship flying through the clouds (and Ghost doing a voiceover if it's a mission). Takes ages... And it's done every single time. Very annoying after a while.
Also, why can't you create teams with people you meet? You have to go back to orbit, then make a team, select a mission and then start again (and each time there are loading screens!). This is not really convenient at all.
Look, I like the game, OK? I have all 3 chars at lvl8, good gear on all of them and like 540 grimoire points. And I'll be most likely preordering. It's just that the game has some annoying things and I think some of the reasons for them is the fact that they also are for the old gen consoles with their paltry specs - that's why I think the game loading is done like it is (I have a PS3 too, OK?). They likely spend the PS4 resources on higher resolution and better textures, without really utilizing its memory size and other technologies in a "modern" way. They should have completely dropped last gen and stuck with PS4/Xbox1 only. But no way Activision would allow that, with the amount of money they spent on the game. "Make the game run on all consoles, no buts" is what Bungie heard