About the online, how much is it important? I'm not a fan of GfWL and i pretty much a single player-only guy.
I think it adds a lot, and I'm usually single player-only as well. Players leave a bunch of messages around the world. Some of the messages are helpful tips, some are helpful warnings, some give encouragement, some tell of their despair, and some of them are straight up trolling you. And you see the bloodstain of other players who have died, and it lets you see the last eight or so seconds of their lives. Once in a while you'll see a slightly visible blue phantom walking around for a few seconds, and that's another player. Sometimes you see other players sitting with you at bonfires. Seeing all those things of the other players makes the world feel slightly more alive, like you're all struggling together for the same thing.
Co-op can help a bunch if you're stuck on a boss, plus it's just really fun teaming up with other players, and helping other players on their bosses. Then there's the invading stuff, which can be pretty scary for new players, but it's also a lot of fun. You only get invaded in human form.