Lennart is among the most visible/vocal developers in the GNOME community, collecting admirers and detracters along the way since he does not appear afraid of voicing controversial thoughts, or even express arrogance.
I first heard of him through a project that was to revolutionise Unix audio, PulseAudio. Using it was not really the best of experiences (I don't yet need any of its advanced features, and it consumed too many CPU cybles), but it may well have just been the way Debian configured it at the time.
Later on, he started work on an init system that was to replace exisiting ones on Linux-based systems, known as systemd. This is among a few FLOSS projects that did genuinely excite me, and for years I was waiting for it to be Debian default. Sadly, too many issues were raised, including the refusal of Lennart to support non-Linux systems, as well as having 'too many features', way beyond being just a traditional init system, worrying many.
Regardless of the controversy, the fact that Lennart was able to shake up the FLOSS world with two major projects like this makes him among the great developers on the planet. He also really put a lot of effort in his projects, portraying great talent while at, as well managing to market his projects very well, at least in my eyes.