I actually have very little experience with SLRs myself. But I love the quality and the ability to use large lenses as I've found it annoying when I see the perfect thing to take pic of, but my point & shoot just couldn't capture it like I wanted to.
I have done a bit of research and I'll try to answer a couple of your questions.
Anyway, the thing with the G1, it also uses smaller lenses too so you have to get lenses made for it(atleast until other companies start to adopt the micro four thirds standard) or you have the choice of getting a four thirds adapter which uses slightly smaller than standard lenses but a little bigger than the G1's. Of course it'll still be a whole lot bigger than your point & shoot, but that's the price you pay for quality.
I've never actually used a DSLR other than trying it at the store, but I thought you would still have to turn on the camera to take any pics. All I knew was that the advantage of the viewfinder was that it didn't use any power unlike an LCD and you saw exactly of what you're taking a picture of. One of my concerns about this camera was the LCD viewfinder, but it seems to be using some special kind of LCD with better picture quality than normal LCDs. As for bootup, DSLRs boot up in like a second or less so I don't think there's much to worry about there other than the slight power use.
Here's a sort of review of a pre-production model with tons of info on it (probably just about everything you'd want to know)
http://www.dpreview.com/Previews/PanasonicG1/