Logo design is easily the hardest thing for a designer to do.
You need to take everything the client wants to say about their company and condense it into one clear and concise package. It not only needs to meet their expectations but also convey this to the general public. I have seen a lot of logos that I think look great, but really fall short in meaning. I have also seen some awful ones that had a solid rationale that really made me rethink my opinion.
I found this great website a while back that showcases a lot of well known (and some not) rebranded logos, complete with a review and design rationale whenever possible. Its a fantastic resource for inspiration and really makes you rethink your own approach to logo design. At least it did for me.