PowerPC software that hasn't been re-compiled for Intel yet. It runs slower and has more "bounces" before it fires up.
If it's pre Tiger apps on OSX, well Apple users would have you believe there are no problems.
I don't know who told you that. No machine has zero problems Macs have fewer than the alternatives.
I have been running Pre-OSX Macintosh applications on my Tiger installations... some from as far back as OS7. I have not had any problems with the vast majority of OSX applications from upgrade to upgrade. The sole exception has been a little utility I like called ASM that has needed a little tweaking by its author to work with each upgrade... but that usually was available within a week of the upgrade's release. I can still run most of the applications that ran on OSX.0 when it was released in 2001.
With the introduction of Leopard, Apple has dropped backwards compatibility with the pre-OSX Apple applications. The only thing I miss is Pagestream... which I can still use on a laptop I have here running Tiger.
Quite frankly, it was nothing short of amazing how few applications broke on the switch from PowerPC architecture to Intel... or how few OS( and lower apps did not work in Classic Mode (usually faster than in the native OS) when Apple switched from Mac OS to OSX in 2001.
Are there some things that break when a Mac upgrade is released? Of course... but they are usually fixed very quickly. Some vocal Mac users complain... vocally.... because they have little patience with the third party application publishers who are not Johnny-on-the-spot with upgraded apps. They post noisy complaints on Mac forums. Those who had no problems didn't bother to post their lack of difficulties for some strange reason.