While playing around with jsfiddle.net as part of investigating bug 1562824, I noticed several problems when stepping around one of the minified click event handlers:
An exception being thrown while trying to get info about an object would cause the manifest to never finish and the replaying process treated as crashed.
When warping to a logpoint for the event, the debugger would show the pause at line 1 of the pretty printed file, instead of the event handler itself.
Stepping in to a call and then back out skips past points in the parent frame that are breakpoints but not step targets, which makes it very hard to step through code that has few step targets (e.g. lots of operations chained together with comma operators).
Fixing these allows the code to be stepped around in pretty easily, and performance feels pretty good.