After years of using it, I've become dependent on the debugger feature, Drop To Frame (described here, here, and here). Briefly, it allows you to select any level (frame) in the call stack during debugging and force the JVM to rollback to that point. It's a little difficult to explain briefly, but trust me when I say that once you've used it you quickly learn to depend on it.
So you can imagine my dismay when I recently started doing all of my Eclipse work on Mac OS X and discovered that Drop To Frame is disabled. It is a feature that not all JVMs support (specifically, those prior to Java 1.4), but I just can't believe that the modern Mac JVMs don't (I've tried running my apps in both Java 5 and Java 6 JVMs).
I asked about this on the Eclipse newsgroups/forums and on IRC, but no response so far. I'm hoping that the blogosphere might have some more insight...
Update: it seems that Drop to Frame is enabled sometimes, but only part-way down the stack, and sometimes not at all. The app I'm debugging has no native code in it, so that's not the culprit in this case. So I'm still looking for some insight into what enables/disables the feature.