In terms of simple and straightforward PP, Lightroom is as good as it gets. You don't have to worry about layers and stuff like that and you get a powerful photo indexing system as well. There's also the benefit of being able to send an image to Gimp (or PS or whatever external editor you have) to do more complex stuff if you wish.
Unlike Gimp, you have to hand over some shekels for Lightroom. There's a free, open source equivalent of Lightroom (called Darktable) that has been written for Linux distros (and now MacOS) but unfortunately not Windows.
I've never used Picasa (free but not open source) but hear that its limited PP stuff is sufficient for most things and you get cataloging/indexing as well.
Perhaps someone who uses Picasa can provide more info.