Unfortunately, as a closed-source propriety platform that doesn't have any support (nor plans any support) for Linux, I had to discount Unity3D almost immediately.
Flash loses out largely for the lack of good accelerated 3D. I've played with papervision but it's just too slow for a anything decent, that I can guarantee will run on even moderately equipped machines.
While I am with you on open source. Unity uses lots of it to build on such as mono.net (C#, boo, javascript), OGG formats, some collada support and FBX support (autodesk), opengl (also directx), Cg formats for HLSL and GLSL shaders etc. I am betting you haven't tried it, if you do I am sure your opinion will change the pipeline and ability to create quality stuff with it is almost immediate. There is not just another product like it currently in terms of pipeline improvements, apis, ide, size of plugin, shader support etc for the web.
