I read Jon Shafer's piece on unlocks, mechanics, and modifiers. Go read it. He makes several very good points, but I'd like to focus on unlocks, and how dangerous they can be to a player's enjoyment of a game if done wrong.
I've recently been playing a city building game called Cities XL from Focus Home Interactive. I've been enjoying it quite a bit, it has a fun traffic model and gives good tools for analyzing and fixing traffic flow in your city. I liked being able to create and then manage my own optimization problem. I'm also told the it is quite beautiful, but my laptop apparently doesn't do it justice.
The game, when played with the standard options, makes heavy use of unlocks. As the city hits new population thresholds, new buildings and road-styles are unlocked, giving the player new opportunities to rebuild and reorganize their city as it grows and thrives. For the most part, these unlocks are awesome and fun as you add more and more complexity to your city; and early on, they come quickly. The last transport option unlocked is highways (500k population). They have double the capacity of your largest road. Since the previous transport unlock was at 100k population, the player is desperately looking forward to the highway unlock. However, they are unusable due to how they interface with the already existing road network.
The details of how and why they don't work are interesting, but not very important. What is critical though, is that it crushed my morale in the game. Here was something I had been looking forward to for hours, and it was effectively useless to me. At first I didn't quite believe it. I spent about an hour trying to get highways properly integrated into my city. After that, I popped online to check out the forums and see what people were saying. Most of them were along the flavor of: "Yeah, highways are terrible, use a mod".
I've put Cities XL down for now. Maybe I'll come back to it later. I got a good deal of enjoyment out of the game, but I can't help but feel that if the highway system had been something worthy of all the anticipation, I would have doubled my total playtime. So yes, unlocks are fun when done correctly, but if you do them wrong you're going to bring the Chick Parabola crashing down upon your player's head.