I used to play COD and bunch of other FPS games before i switched to BL. When i started BL (1) the health bar came as a kind of new thing to me despite playing a lot of games that have health bar. The reason was simple, most recent games switch to splash based health. When you take damage the screen turns red and after a few moment fades away. This also makes the game slightly easy because you can jump out of cover, shoot and then duck back when taking damage and repeat the same thing once you are healed. Same can't be done in BL because the health doesn't regenerate on its own (ok it can be done with skills, class mod, etc). There is the shield to solve this problem but this rarely helps in BL because unlike other FPS games where you can use an assault rifle as a sniper, same is not true for BL because of the low damage, low fire rate and worse of all the accuracy. Unless you close the distance, most shot simply miss. Get close and you risk getting shot and killed while running between cover. And over that all, enemies have shield which can absorb quite a bit of punishment before going down.
I was a casual player of COD and this explains why i found BL1 so damn difficult. Unless you mean business, you die. The game doesn't cheat because everything else sucks. The guns, health and the enemies. Play seriously or end up in a thrashcan. The levels, enemy ranks and weapons (specially boss's trademark weapons) made the game even more difficult... BUT any player who was arrogant enough to hold on to it, got rewarded. got paid, handsomely. You got lots of guns, lots of ammo, and a way lot of bad guys. Shoot and enjoy. It was like paradise for FPS lovers, specially those who are used to retro style games. BL is not a retro game but it had retro style difficulty at places, specially solo. You learn the game by killing and dying.
It won't be fair to compare BL to other FPS because BL is a RPG shooter i.e. skills to upgrade your style and powers whereas BF/COD is plain shooter. You shoot and kill the same kind of guys over and over again, in SP campagin. Whereas in BL (and way more in BL2) you get variety of enemies. Even sub variety. Each with its own health and attack pattern. This was why most people who played BL ended up as addicts. You get RPG but without the complexity in choosing lines rather the complexity of enemies. You can either kill badass and then use the smaller enemies to gain second wind if you get downed or clear the smaller enemies first so they can't distract you and then take on the big guy.
BL2 was kind of a dream come true. Whereas most sequel increased the complexity by adding unnecessary stuff, BL2 increased the complexity by adding more enemy types and variants. But also by removing repeaters and clearing up the charts. The inventory of BL1 looked more like a debugging tool than an inventory for sorting guns and items. The move to highlight whats skills were getting boost was also a major point. Last but not the least the removal of guns and shields that allow health regeneration was a big surprise as well as a way to challenge players into stop the hide game. There are still shields that allow health regeneration but better when damaged i.e. you are forced into a battle to regenerate health. It was a risk vs reward system. Same for Moxxi's guns. Without fighting enemies, that gun is useless.
I wonder what BL3 will bring to that table but i am sure, it'll retain the thin red line because thats what make Borderlands a real Borderlands. Thanks for reading.