Well I won't get into the obvious different between commercial (dedicated team who's careers it is to work on it) and open source (various people who work on it in their spare time) the effects of that speak for themselves so that's off the table from a technical stand-point.

My main issue with Lazarus is the Mac OS X port. I love it on Windows, it does a decent job, but on the Mac it fails miserably in the stability department. Windows it's better, but there are still issues. I'll try to list them as I know them.

Mac OS X Issues:

GDB debugger seems to crash all the time on my system (less than a year old iMac with the latest version of Snow Leopard) and other than blindly compiling/building I cannot debug my source with it. I've tried fixing it, but nothing seems to do the trick. This is a problem for the last stable release and even some of the more recent SVN versions too.

There is a thread about another develop having the same issue, but when he stores his source on an NTFS drive. You'd have to go to the Lazarus forums to get more details on that. It did however seem similar to the issues I was having. Forum Thread

Beyond that, obviously because Lazarus uses GDB for debugging, instead of a much clearer debugger error message you sometimes get SIGSEV errors that merely tell you that you screwed up with memory management. This is common among all platforms. I remember Delphi's own in-suite debugger was a bit more helpful in this matter.

There is a project for a Lazarus/FPC specifically designed debugger, but it's still a long way away from being as nice as Delphi's or even GBD in a way.

Cross-platform Advantage:

Delphi doesn't yet have it, but sometime this year we'll be receiving a 64-bit Windows and a Mac OS X edition. Still this doesn't give us support for iOS yet. You can use Delphi Prism with Mono and such libraries as MonoTouch and XNATouch, it has been done, but there will never be as nice as a more closer to 'native' solution.

FPC on the other hand is iOS capable and will allow you to make iOS Apps through Xcode. I don't think this can be done with Lazarus just yet.