Mods expand the game immensely. "Vanilla" Minecraft is certainly fun, but not enough fun to hold my attention for long periods of time. When you add mods to the game, there's a lot more things going on and a lot more things to do. Take a mod like GregTech Intergalactical - look at the wiki page -
http://ftbwiki.org/GregTech - really look at it, scroll down the entire page and look at all those items added to "Vanilla" Minecraft. Thousands!! And that's just one mod - granted, it may be the biggest mod of all, but it's still just one mod of many mods available to add to the game. Each mod typically adds 100's of things to the game, and that is exactly what was required for the game to hold my attention for a time frame measured in years. I started with Vanilla Minecraft, we probably all did, and it was awesome fun for the first few months, but knowing myself I am certain I would not be playing it after the first year. I would have gotten bored with it because I don't have a huge desire to build stuff. I like to explore and figure out new mods and new machines and mine the new ores and process them and see what they can make... etc. I never get bored with modded Minecraft, it seems there is always something new to do, something fresh and exciting. Notch and Mojang gets all the credit for making this fun game, but the "mod community" gets all my credit for helping to keep my interest in the game going.
This is the perfect example of why I feel "open source" is the only way for humans to progress. If Mojang had locked the game up and refused to allow modding, it would have put such a sour damper on the gaming world. We would have none of this fun we're having. I wish other companies realized this fact, but of course the almighty dollar (the prophet of profit) rules our world. And on that note, I will quit typing and climb down off my soapbox. LOL (Microsoft? hahahaha That's a good one! sigh)
