Some Lessons Learned in Dark Souls

Don't worry. You get this armor... you just have to wait until the very end of the game to get it!

So I've played Dark Souls for well over a hundred hours. I've (somehow) completed the game twice and I've made several observations I'd like to share with you. These may especially be helpful for new players. Minor spoilers may follow.

1. Decide what kind of character you want to play EARLY. This is a mistake commonly made in RPGs. You just got the game and you want to experiment when every single little mechanic the game has to offer. You want to swing the biggest swords and also cast spectacular magic. Dark Souls definitely allows a lot of flexibility, but you have to be careful about overextending. I've found that picking two of the stats that affect damage: strength, dexterity, intelligence, and faith, and developing only them is far better than developing all of them evenly. Also, decide what kind of weapon you want to wield early. In Dark Souls, your strength and dexterity (as well as intelligence and faith if you decide to wield magic weapons) scores decide what kind of weapons you can competently wield. If you want a giant axe make sure you have lots of strength. 
2. Resist the temptation to increase the Resistance stat. Since increasing any stat adds to your Resistance score, there's really no point to increasing it on its own. 
3. NPC death is permanent. If you accidentally, or intentionally, kill an NPC, they ain't comin' back. Also, if you accidentally damage an NPC, they will pursue and attack you. However, if you get away you can request "absolution" from Oswald of Carim, for a price, and set things right again. He's located on the roof of the church in room you enter after the Bell Gargoyle fight. 
4. Bosses are always easier with friends. If you're having trouble with a boss, go look for help! Players will typically leave summon signs near bonfires and other designated areas. The game also provides help through summonable NPCs such as Knight Solaire. Solaire will become your best friend throughout the game as he turns difficult bosses into not-as-difficult bosses. Remember, you have to be human to summon a phantom! 
5. Get lots of Humanity! You start off the game as "hollow". Bereft of Humanity, you're more susceptible to damage, you can't experience Dark Soul's online features, and you look like a zombie. Pretty soon, you'll be picking up an item called Humanity. If you use a Humanity, the 00 in the upper left hand corner of your HUD will start to count up. You can use that Humanity at a bonfire to kindle the fire (gives you more Estus Flask charges for healing, also assuming you've learned the Rite of Kindling) and you can select "Reverse Hollowing" to restore yourself. When human, you take less damage and you can summon phantoms/invade other worlds. Just Google "humanity farming" for some good farming spots.
6. Do NOT use all your Humanity! Don't use all your Humanity as you acquire it. Leave plenty of it in your inventory in case of emergency. Let's say you die inside a particularly difficult boss's room: if you don't have any Humanity in your inventory, you can't reverse hollowing and summon help. In other words, you're shit-out-of-luck.
7. Spend acquired souls ASAP. The first time I played Demon's Souls, I lost around 40k souls at the beginning of the game from being an idiot. When you die in Demon's Souls/Dark Souls, you drop all your souls (and Humanity in Dark Souls case) and have to return to the point of your death to get them back. If you've decided on what kind of character you want to build, you shouldn't have a problem spending all your souls as soon as possible on leveling up, weapons, etc. 
8. Avoid the Blade of the Darkmoon covenant until you're a competent PvPer. Covenants grant the player extra items and weapons upon joining. The concept of the Blade of the Darkmoon covenant is awesome. Your job is to get revenge on people who have committed "crimes" such as betraying their covenant. So, if you equip your covenant specific item and walk around, eventually you'll be summoned to another world as a "spirit of vengeance". The problem is that most of the time you'll be summoned to Anor Londo and be annihilated by much higher level assholes that are waiting for you. Until you're comfortable taking on multiple opponents in PvP, stay away. 
9. Do not enter Anor Londo unless you have a decent stock of Humanity. After ringing both Bells of Awakening and conquering Sen's Fortress, you are flown to Anor Londo to retrieve the Lordvessel. The boss(es) of Anor Londo are an annoying pair of knights known as Smough and Ornstein. You fight both of them simultaneously: 2 on 1. That is unless you have help. Summoning Knight Solaire is the only way I could complete this fight the first time through. The second time I came close to doing it alone, but it's still extremely hard for me. If you don't have Humanity, you can't reverse hollowing and summon Knight Solaire or another player to help. Plan ahead! You can leave Anor Londo before getting the Lordvessel, but you have to go all the way back through Sen's Fortress and, believe me, one trip through the Fortress is enough. 
10. When you are human, beware of invaders. In Anor Londo and the game's final area, I was invaded so frequently that I eventually just went offline so I could farm Silver/Black Knights in peace. Invaders are not to be trifled with. Most invaders sport powerful PvP builds and, if you're just playing PvE like me, odds are you're not optimally equipped to defeat them. They can be cheap, too. Nothing is more annoying then being killed by an invader and then making a stupid mistake on the way back to pick up your souls and losing all of them. If invaders get annoying, go offline. Or stay hollow. 
11. Never give up. Dark Souls is a hard game, for sure, but it's not impossible... well, except maybe Smough and Ornstein with no help. But besides that, you can do it! Believe in yourself and all that jazz.