This is one of the few games that are really *perfect*, IMHO.

But why? Because
- it's simple to understand and learn (few rules, few buttons)
- it's easily scalable in difficulty by modifying the speed or showing a preview of the next block (or not).
- it's a successful marriage between reaction-based games and puzzle games (higher speeds shift it more into the twitch area, though)
- it doesn't need a high-powered system (no high cpu speeds, not much memory). A simple 8-bit machine with "some kind" of text mode display suffice - it doesn't need hi-res graphics and it doesn't even need sound. In fact it's not that easy to find games that demand less.

and most importantly:
- it appeals to the "I need to put things in order"-compulsion and the "I like to fill empty spaces"-feeling
(which may depend on your socialization, of course ;-)

@"Alexey Pajitnov"
Why should anybody have him murdered? The soviet state earned money because of him (not just "money" but foreign currency!) and because of the emerging glasnost policy in 1985 everybody had more important problems anyway.

take care,
Calibrator