Pros: Unparalleled amount of content, amazing character development, phenomenal role playing, everything
Give this article a read. It's really informative about how DA:O is the most ambitious game. Not only is it the most ambitious and most complete, it does everything it sets out to do almost perfectly, with very little flaws. This game is really first in line when talking about the best of all time.
This game was in development for a long time. Bioware is most well known for Mass Effect, KOTOR, Baldur's Gate, and somehow Dragon Age is overshadowed by both Mass Effect and even The Witcher. Not to take anything away from those games, but Bioware's main focus are character development and lore. DAO does an even better job creating characters than Mass Effect, and has much better plot build up.
Dragon Age: Origins has some amazing locations. Places aren't reused and repetitive like Mass Effect and Dragon Age 2, and the game isn't linear like The Witcher. One playthrough takes about 50-60 hours if done properly, and one of the most unique features about the game is how there are six playable, 2-3 hour "Origins" which are essentially the backstory of your character, showing how and why he became the protagonist of the game. Which origin you get is dependent on your race and class, when creating a character. This is a remarkable feature, indeed.
DAO is one of the best RPGs in recent years, being very heavy on RPG elements, both in more old-school terms (stat-based combat, heavy stat emphasis on items, strict classes), and in more modern ways (how decisions affect character dialogue/attitude and how quests play out). These more "modern" RPG elements are taken to an extent only seen in Bioware titles like Mass Effect and Dragon Age 2, but it surpasses those.
Combat is like the classics, without any of the clunkiness, though it's much more simplified. DAO emphasizes party-based, strategic pause and play combat. It feels very much like KOTOR, and has many similarities to the combat in Neverwinter Nights, Icewind Dale, Planescape: Torment, etc. Don't expect the same level of complexity or playstyles though, Dragon Age only has 3 classes (Warrior, Mage, Rogue), but with a few unique specializations for each. Strategy is so important in this game... poor strategy combined with taking on higher level enemies will make victory pretty much impossible. This turns off some of the small minded typical gamers, but is great to see if you like a challenging game.
Dragon Age: Origins works on every level: as a character driven role-playing game, an adventure game, and it just does a phenomenal job at creating a world. It's huge in scale yet attention to detail is second to none, and it has no major flaws. It also has excellent mod support. DAO is a once in a lifetime game, don't expect to ever see anything else like it.
Install these mods before starting a new game: