How long have you been using an Apple?
OS X when it first came out was coded new from the ground up. Even the compiler was new. As OS X matured the code was cleaned up and the compilers became more efficient. The result was a faster overall experience. However OS X did have to leave a few old legacy machines behind as well as legacy software.
When Jaguar first appeared, Apple had to fight off a class action lawsuit from it's user base because it wouldn't fully support quartz with the 1 - 2 year old 16mb graphics cards. Apple did finally get quartz to run crippled on these machines but they certainly didn't run the OS GUI elements faster.
Now Apple is at a cross-roads again, they are moving on to 64bit G5 processors. They want to optimize the code and compilers for the future and not look back. They have a chance to take a commanding lead in this market before Microsoft can get out a good 64 bit OS themselves. Apple wants to add gui elements that need at least 64 mb graphics cards and take the lead in this area as well.
Even if they sell 10 million mac minis, Apple is not going to let the mini define their cutting edge. This mini defines the minimum configuration for the future. It definitely runs panther well, and it will probably run the first release of Tiger at an acceptable level. However It will not run faster and faster with each release.
In fact I wouldn't be surprized if Apple updates the mini with 512mb of ram, and a 64mb GPU about the same time that Tiger is released in 5 months.