I've been gaming since I was a small child, and this is, without a doubt, my favorite game.
Many consider Dark Souls to be an extremely difficult game, but honestly, I find many other games to be much harder (I play many games on easy mode, shameful I know). Your Dark Souls experience will likely be determined by how quickly you can learn a game and its mechanics. A couple things that build upon its difficulty is the lack of conventional checkpoints, and specific dangers that require you to "suit up" properly. Bonfires serve as your checkpoints, and they are few and far between, some of them are even hidden behind illusory walls or a drop off a ledge. You may need to outfit yourself for a certain resistance, or carefully engineer your outfit to get the most defense without suffering movement penalties. The control is very well engineered, and after you learn your character's movements it's not unreasonable to be able to run through entire areas without even getting hit; which is good, because getting hit hurts. A lot. Unless you go all-out defense or focus on leveling, many bosses in the game can one-shot your character or combo them in a few quick swipes. Every enemy and boss is readable, and the most dangerous moves are telegraphed well, making defensive play a priority. Dark Souls will reward patience and careful planning.
Online play in Dark Souls is equally dangerous. If you're in human form, you can be invaded by another player at any time. Likewise, you can invade others or summon help from other players. There are unofficial PVP hot-spots, and the new content comes with a PVP arena that supports duels, team battles, and free-for-all matches. Arguably, this is the only area in the game I feel they can improve greatly on, for some reason the PC version doesn't connect players very quickly. Covenants play an important role, while each one has a specific goal regarding PVP. You can help players beat bosses, you can invade the world of an owner of a dragon scale, you can invade the world of those who trespass in Darkroot Garden, and there are many more covenants.
The growth concept centers around Souls, which you get from items, enemies, and bosses. You can use souls to level up, buy items, and repair or upgrade your equipment. Another growth concept lies in Humanity, tiny dark sprites that you collect after surviving an invasion or using an item. You use humanity to reverse your hollowing, turning you into a human form, which in turn lets you kindle a bonfire or enjoy many of Dark Souls' PVP aspects. The more humanity you have, the greater enemies' drop rates will be. If you should perish, you lose all of your humanity and your souls. If you can make it back to the very place you died or fell from, you can collect your dropped souls and humanity in its entirety. I've lost up to 200,000 souls at one time, but by that point, the game had taught me to shrug off defeat.
The highlight of the game is its boss battles. Most bosses have their own memorable music theme, and many of them are absolutely huge. A highlight for me was the boss battle between the Great Grey Wolf Sif, a sword-wielding giant wolf who guards his master's grave. Most bosses relinquish a specific boss soul when defeated, which can be used to create unique weapons or shields. Some bosses are simply huge monsters, and others have an important role in the world.
The game auto-saves constantly, with the goal being forcing the player to live with their mistakes. There's a certain NPC you can free that will eventually cause you trouble, and you may find yourself being extremely careful putting your controller down, as to not accidentally hit one of your triggers to attack an NPC and make an enemy of them. If you wanted to, you can fight nearly every NPC in the game. Definitely not recommended until you're off to beat the game, however.
The PC version of the game has its quirks. A resolution lock can be fixed by a user-created mod, which I personally recommend. There are keyboard controls, but this game demands a controller (I've been using a usb xbox 360 wired controller). The new content is great, and sheds a little more light about a few of the more obscure characters in the game, one of which is only mentioned in texts.
Dark Souls is, without a doubt, my favorite game of all time, and I like a lot of games. It rewards learning, has an abundance of options available for character builds and tactics, a very large amount of items and equipment, and most of all, it has a dark, beautiful and frightening world and tells the most subtle of stories. Don't let the promise of difficulty drive you away. This is a must-play.