I got stuck with a boss (not sure if 3rd of 2nd), and found that having to repeat a big chunk and collect the stuff again pretty frustrating.
How does it work now? Do you revive at the start of the level but keeping all the key loot? Or did they include checkpoints?
It's split into 2 sections. Levels 1 to 3 and Levels 4 to 6 the only checkpoint in the game is between stage 3 and 4. You have to complete these entire sections without dying.
At first if you turned the console off to go to bed or whatever in the middle of run it would reset to the beginning of the section you were on the next time you played (either level 1 or 4).They later allowed you save your game in the middle of run. Again if you die at any point you reset to the beginning of the section and your save is gone.
Regarding upgrades you permanently unlock key abilities like the sword and grappling hook which make the combat and traversal easier.
Weapons are a bit trickier to explain. You will always start with the pistol however you find other weapons randomly in the game. In stage 1 you will only find the carbine and the shotgun. I think in stage 3 you unlock the rocket launcher. Once you've got further enough to unlock a new weapon it will start appearing in the earlier stages e.g. the rocket launcher will now appear in stage 1. The later weapons are usually more powerful making the earlier sections easier.
BUUUUTTT that's not all. Each weapon has a level, a base ability and a trait you can unlock! The level is standard the higher the better and base ability is what you would expect from each weapon. The traits are what make this game great. Traits are basically sub weapons within each weapon class E.g the standard rocket launcher fires a burst of 3 missiles however traits for it include Mega Rocket (Replace the 3 rockets with 1 really powerful rocket) and full auto (You fire about 10 at once!) and there are many others just for the Rocket Laucher
Some weapons you pick up have traits. You will initially need to kill a certain number of enemies to unlock the trait but once unlocked it too will start appearing in any stage of the game.
So as example the Full Auto Rocket Launcher is very powerful if you get it early in stage 1 you will nuke everything. You initially need to get further enough to unlock the rocket launcher (stage 3). You then need to fid another Rocket Launcher with the weapon trait Full Auto. Kill a certain amount of enemies to unlock Full Auto, Full Auto Rocket Launcher now has the potential to appear anywhere within the game, Win.
Honestly the Full Auto Rocket Launcher is somewhat middle of the road, Some of the late game guns make the BFG look timid.
Hope all that waffle helps. If you need anything clarified or want to know more just fire away.