There are a couple of things that make an RPG a success in my opinion. There are tons of RPGs on the market, but few of them rise to the status of greatness. Greatness required innovation. The recreation of previous greatness is simply forgery. Sometimes a game can get it right the first time.Which is especially true for groundbreaking games. Call of Cthulhu is an example of a game that got it right the first time. Sandy Petersen is a great designer for one. And previously no game had really truly addressed the Lovecraft theme previously. Dungeons & Dragons is great up to 2e. And Advanced Dungeons & Dragons / Basic / Expert are really an era more than a single game following after Original D&D where things were still largely great because the ethos of the game hadn't changed much. GURPS was great up to about 3rd edition. RuneQuest was great due to Glorantha. But this post isn't a roll call of greatness. This is instead about why C&C is great. I would submit three reasons in this regard.
Reason Number 1 Why Castles & Crusades is Great: It's simple. The Seige Mechanic is straighforward, intuitive, flexible and easy to pick up. You can roll up and play C&C in about 15 minutes. No muss no fuss. It allows CKs to run a game largely without books too. Don't get me wrong the Troll Lords put out some awesome books and supplemental material. But you don't need them! If you can recall what hit die each class gets, roll up six stats, and understand the basics of the Seige mechanic then *poof* play! Sure there are racial bonuses and the like and you may want a pre priced equipment list, but really you don;t need any of that. Just a strong desire to have some fun and have some adventurous times.
Reason Number 2 Why Castles & Crusades is Great: Good storytelling. The C&C guys are great at telling engaging fantasy stories. I like to create my own worlds by and large. The only real world I ever used stock as it were was Greyhawk. But after reading the depth of story associated with Erde I have seriously considered using it as my new campaign world for the next several years if not more. I was especially moved by the story of Luther and the Sea of Dreams, but that is just one tale amidst a tapestry of writing that spans all of their handiwork. C&C isn't about just spitting out a torrent of new worlds for you to buy. They have invested over 15 years to this campaign world and it is rich and deep. It is a world that literally begs to be played.
Reason Number 3 why Castles & Crusades is Great: The people. Stephen and Davis Chenault are simply the most genuine and real people I have yet met in the professional RPG world. I'm not downing anybody else here, just praising these two guys and those who work with them as darn good people. Stephen answers his own phone, emails his customers and he and Davis are guys anybody would want at their table. I know people have said their staff is too small, their production dates are screwed up, their editing could be better. But do we really want an overstaffed impersonal crew working at the front office? Absolutely not. Stephen and Davis are first and foremost Gamers. They think like gamers and act like gamers, nice ones anyway. I've been hanging out at a lot of gaming communities lately. Game specific ones and more generic gamer havens. And none--and I mean NONE--are as friendly as the Troll Lords site, fora and the people who play C&C. Some others are downright mean and exclusive. I always feel like I'm on trial when I'm at those places. But the Troll Lords and the C&C community is a place any gamer can feel at home. And they don;t push their views on you. C&C is a very open and flexible game. Alot of it is up to personal interpretation, which is another reason it is so great. But noone has an agenda to push, other than they really like C&C.
For all these reasons C&C is also the perfect game for beginners. Easy to learn, easy to play, very flexible and forgiving. Great product support if you are looking for that, and a warm and fun loving community. What more could a new gamer ask for? Heck what more could any gamer ask for?