In your opinion, what makes games fun?

In my experience, the act of completing any kind of goal, whether self-created or preprogrammed. It doesn't matter what the goal is or what you've accomplished, instead it's the fact that you've received something in return for your efforts. In a game like World of Warcraft, you earn a new ability or new stat benefits for raising your experience. In Magic: The Gathering, you've compiled enough mana to cast a certain spell. In Minecraft, you set your own goals ("I'm going to build a mining outpost near that cave today") but receive the same satisfaction for completing them. In all of these instances, the return on investment, no matter how big or small, prompts me to continue playing and enjoying all that the game has to offer.

In addition to the goal-setting mentioned above, there are a few things that must be present for me to fully enjoy a game. One is a gripping/compelling/entertaining story. While I acknowledge that not all games need a story to be fun to play (chess, poker, Mario Kart), the presence of a guiding narrative helps me immerse myself into the game more fully. The majority of games place you right in the center of the story, usually having you play as the protagonist and experiencing a narrative on your own. Others place you in the center of the story, but produce additional storytelling material to tell more tales from different angles, fleshing out the world and making it seem like the world you're in is alive and there are things happening that you're not bearing witness to. Some even put the story in pieces and let you discover the narrative on your own.
The final thing that really gets me into a game is good art direction. Seriously. I spend a lot of time looking at what kind of creative work goes into big-name and small-name games, and the amount is pretty consistent. There's a lot of creative work that goes unnoticed by most, but it's something that really excites me about games and makes me feel like the designers really put their full creative effort into building an exciting world to play in (of course, this isn't the case for most board and card games).

I <3 Bees: What made it fun?

There were a lot of things that made I <3 Bees fun. But they could likely be boiled down to a few things: individual contribution, a large set of challenges, and immense return on investment. Though it took the collaborative effort of several hundred thousand individuals to complete the game, each on contributed to the problem-solving process in some way. This enabled every part of the group to feel like they had a part in solving the problem, an experience which lends immense satisfaction and a sense of pride. The large set of challenges prompted the group to explore every hypothesis and probable solution, a situation which allowed for every player to contribute something to the collective. And finally, the immense return on investment was the completion of one set of challenges and unlocking the next, which incited the collective to continue working together.
In addition, a couple of the things I mentioned above were present in the I <3 Bees game. The goals and completion thereof, and the gripping story. The reward for completing the goals was unlocking another small piece of the story, up to the point where players were putting together their own narratives based on the collective knowledge base.
In my opinion, the I <3 Bees game was a near-perfect massively-multiplayer game. It allowed for every player, regardless of skill level, to contribute to a mystery that seemed to have real-world implications. In the end, it ended up being a testament to what people can accomplish when rallied under a guiding task.