Why another open source Flash Player? Couldn't you just join the Gnash project?
Lightspark started as an experiment in achieving high performance by exploiting the features offered by modern hardware. Such target is nearly impossible to reach working inside an existing and large project such as Gnash. As anyway Gnash performs ok on older clips (those not using AVM2/ActionScript 3) currently lightspark is able to fallback on Gnash to handle them.
Lightspark tries to do everything in the right way by following the specifications released by Adobe. I also think that Flash is a technology that is here to stay for a long time, both for market and technological reasons, so a well made (read: not an hack) open source implementation is a investment for the open source community.
Why are Pulseaudio/FFMpeg needed? Couldn't you just use GStreamer to handle all the audio, video and synchronization work?
GStreamer is a complete media playback solution (actually even more) and it's the right choice when developing a media player. But Flash is not a media player. Flash allows client code to apply a variety of effects/transformations on both video and audio. This means that lightspark needs access to the raw decoded video/audio frames and must handle the presentation of them itself to gain the needed flexibility.
Lightspark causes lockup or system unresponsiveness with ATI radeon cards
Lightspark uses some rather advanced OpenGL techniques that requires good support from video drivers. Issues has been reported with the open source radeon driver, but could be easily fixed by upgrading to Mesa 7.8.2 or better. Report a bug if after upgrading the problem persists.