Actually, Heather, Picasa is also non-destructive. It saves all your edits then applies them to the original as it renders it to screen. If you then click on the "Save changes" button at the top of any album, Picasa will save the edited versions as JPGs, BUT it will save the originals in a subfolder called - you guessed it - "Originals"
Of course, if you then do MORE edits, and then save AGAIN, your original originals WILL be overwritten by the "new" originals - if you see what I mean.
I tend to do ALL my edits, save the changes, then copy the "Originals" folder off to something else, like a DVD. And, of course, I do keep a backup folder of original, untouched photos directly imported off the SD card, and I NEVER work on THAT folder - only a copy of the folder. Hard drive space is cheap these days, as are blank DVDs.
I'm not trying to compare the two products in terms of all the other things - of COURSE Lightroom is a much more "pro" product, and will do a heap of stuff that Picasa won't. But so will Aperture, and it is almost as fast as Picasa.
My post, and "comparison" were purely regarding the speed to screen, and the responsiveness, or lack of it, between the two programs.
I just returned from a day out at sea whalewatching, with several hundred photos, all in DNG RAW (Pentax K20D using bursts of continuous-mode shooting. I started to process them in Lightroom, but it was like wading through mud. It was taking forever to do anything. Just as an exercise, I switched to Picasa, imported the entire 16 Gb worth of photos, sorted them, tweaked them, one click of sharpening, and saved the keepers as both 8-bit and 12-bit JPGs which I then burned to DVD for my brother to take home to the UK. All of this took about one hour, with Picasa not skipping a beat and feeling lightning fast.
The next morning, I did the same thing, but using Lightroom, as a more definitive process, rather than the "quick and dirty" one I did for my brother. I ran out of patience after about 2 hours, with everything feeling so slow, Lightroom crashing twice, and the end results being no better to my eyes than those I did the night before.
I don't hate Lightroom - I use it. But I do hate its lack of speed and responsiveness, even on a very high-spec machine. That was my main point. What is it doing that Picasa isn't, that slows down its screen rendering so much?