There's a very good movie called 'Wonder Boys' where Michael Douglas plays an English Professor who wrote a great book 20 years ago and has been coasting ever since. He lends a draft of his new book to the coed he's sleeping with and she is confused by it. She says, 'I don't understand...I'm reading this and it's like you didn't make any decisions at all'.
That's how I feel about CIV. 'Let's have a game with guys with sharp sticks! And Muskets! And Nuclear Submarines! And Canoes! And the Sistene Chapel! And Helicopters! And on and on and on...'.
So the game is so huge and unwieldy and changes so much every 4 hours it's like a different game. And I hate the 'it will take 18 hours to finish this game' thing.
But I admire Civ4 and played it for about 40 hours before I decided I hated it.

So will I love Colonization? I never played the old one. It's more focused on one time, right? What makes it more than just the middle 3 hours of a regular game of CIV4?
OK i haven't plowed through all the responses so I do not know if you had your answer yet, but Ill try to explain the difference between civ and colonization.
Where civ focusses on techs, col focusses on population. You start out with nothing but a boat, a few guns and a 'settler' and you should settle down, deal with natives and other european powers and the crown back in Europe.
The challenge in col is gaining independence from the home country. You can loot and plunder the riches of the natives if you want, but the focus is on building prospering colonies, not on combat like in a civ game. (Don't haunt me to say the focus of civ is not on combat, it sort of is unless you get the map and other civs just right.)
To gain independence, you must become completely self-sufficient. That means you will have to teach professions to new colonists that are born, you must process raw materials like cotton, sugar and furs into cloth, rum and coats and sell it back in Europe. It means you will need to make your own muskets and artillery to fend off the royal army that comes to opress you once you claim independence.
The focus of col is on building colonies and them make them self sufficient, until finally you can claim independence. Until that goal is reached you have to deal with a greedy power back home that keeps raising taxes. You will have to deal with other european powers that send privateers after your ships to steal your valuable, hard earned goods. You sometimes have to wage war to grab land from another nation or to defend your own land from being claimed.
there are no techs really, just colonists of all kinds. Some aren't specialised in anything, some are. Specialists always haul from Europe. You can train your own once you get schoolhouses in the New World, but you will need schools and a teacher first. So before you can work your fields with expert farmers, fish with expert fisherman and use process tons of ore with your blacksmiths, you first need to gain the knowledge on how to do that from Europe. Once you have a specialist in the New World, he can instruct the unspecialised colonists in his profession.
There is also the matter of the founding fathers, which resembles the civ mechanics the most. You need to produce 'liberty bells' in your colonies. Liberty bells help see your people that independence is a good step forward, so that if enough people are cionvinced you can finally claim independence. Liberty bells also help convince founding fathers to join your congress. Founding fathers give you a bonus in certain things. For example, Francis Drake makes it so that your privateers have 50% bonus combat strength. There are 5 types of founding father, specialising in politics, trade, religion, exploration and military. When a new ff joined your congress, the game presents you with new ff's to pick, one in each category.
The game focusses heavily on infrastructure, making it a game that lasts about as long as a civ game. Specialising your infrastructure properly requires a lot of micro management, but I guess with the new engine it will run smoothly.
Well, I do not know what else to tell you. Should you have questions about game mechanics or how stuff works, just let me know and I'll try to help.