Regarding concerns about the download size of Tidalis: I just can't fathom that as the issue, in the main. It's not like we advertise that it's a 400mb download. What do folks imagine is happening -- that people start downloading it, go "oh noes, 400mb" and stop the download instantly? I think that once someone starts downloading something, they're pretty much committed to it unless it's going to be some ridiculous amount of time to get (like days, on dialup).
Granted, it's not doing us favors in terms of capitalizing on their attention immediately, but the reason that the Tidalis game is so large is simple: the art is power of two larger than that in popcap games, and the music is very high quality oggs, rather than what I'm assuming are some sort of synth-based soundsystem (midi, tracker of some sorts, etc).
You do know that the popcap games only run at 640x480, right? Or at least that's true for PvZ. Maybe there isn't desire for a casual puzzle game with high-res graphics and music, but honestly I don't think that's the problem here.