Easily the RPG of the century - missing this experience is a tragedy for those who love fantasy RPGs.
Dragon Age Origins is an incredibly unique experience; you start out with one of six Origins stories, depending on the race and background you choose. Once you get to the portion of the story that is common to all characters, the main story quest is similar, but how people approach you, treat you, talk to you, and your responses in return all depend on who you are. There is no right or wrong, no obvious good or evil - you are faced with many decisions, both small and large, that can greatly impact the world around you, one way or the other.
Throughout exploration of the game, you will come across codex entries that explain a bit about the game history, different characters, controls, tactics, quests and tidbits of interesting information specifically to an area or subject. These are all stored for your viewing pleasure, which comes in very handy should you need them as reference for a quest in the future. You also have a map which details places you've been and is always accessible; and often shows areas of interest pertaining to your current quest(s). Your current and completed quests are also shown, and are updated constantly, making for very handy reference.
The battle system is excellent, intelligent and thoughtful. You can customize your character according to class, and what weapons style you prefer, and then (provided you unlock them) gain access to two further specialization classes as you level. Your party members are controllable by a quick touch of the right or left bumper, and you can either choose to play as them, or issue them a quick command and then move on. Alternatively, once you become more accustomed to the battle system, there is a wonderful thing called tactics, in which you can control each party member's actions in battle by issuing them a series of specific commands in the tactics menu - a valuable system which i used and changed according to the situation, quite often. You can also pause-and-play to activate spells, techniques or use a potion or item.
Although there will be times you get stuck and possibly frustrated with the seemingly sudden difficulty spike, often you can leave freely and come back when re-equipped, or just re-assess your tactics to fit the situation. It makes you think, it makes you consider all your options, and when you do achieve victory over a tough spot, it is a very good feeling indeed.
Another thing about battle is that there are essentially no re-spawns; if you leave a difficult area to restock and come back, you aren't going to have to redo the entire area to return to where you were. Obviously there are times (dependent on plot) where you can't leave but usually you can, and not having to redo areas was very refreshing.
As far as party members are concerned, you have the option to recruit them if you wish, or let them go anytime. Your actions will affect their opinion of you, and often they will disagree with eachother, so it can be a challenge to please everyone. If they come to like you enough, you may receive bonuses to their skills, but if you fall in their favour, you will lose those skills, and possibly them as well if they dislike you enough.
The possibilities are seemingly endless... this game has immense replay value, and is time and money well worth spending, especially if you are an RPG fan. Bonuses are that there is already, and will continue to be, support through dlc and a sequel already in development, so there's no fear of getting bored :)
The world is deep, the story is rich and engaging, and there is so much to discover, you won't be able to wait to find out what happens next....