Use 2+ monitors and 2+ desktops ('Spaces' or whatever they're called). When you're expecting to go from app A to app B, instead you're taken to app C. Or, you're taken to an unexpected window of App B on a different desktop.

Also, the Z-ordering of windows seems really odd. If you have multiple windows open from two apps, and you ⌘+` between them, sometimes all of App A windows are brought to the front and App B windows are hidden. Other times only one App A window is brought up. Meanwhile, if you have App C chilling on a different monitor, it may or may not disappear at random.

Even if it worked as intended, it would be flawed when compared with Alt+Tab on windows.

I agree, the full-screen and multi-monitor workflow stuff in OS X has been improving slightly, but it's still quite a mess. Apple seems to go out of their way to make it not work well.

That's very generous. I think a more correct response would be that the multi-monitor workflow in OSX is shockingly bad, so much so that we have to question whether anyone at apple uses 3 or more monitors as their work environment at all.

Is there some company edict, or cultural pressure that discourages it ? I don't see how any of these issues could have survived early-alpha (of Lion, and ML, and Mav., etc.)

Oh I see. I frequently use a second monitor and there are times when some app will open or show that really had no reason to. This tends to happen when I do ⌘+tab. On the other hand, window navigation for me has been fairly decent.