Well, that's iPhoto for you. The more I learn about it, the less I like it.
With Lightroom, you control the location/structure/hierarchy of your image library. Personally, I have a master photo folder, then a subfolder for each major type of work (portraits, sports, cars, etc), then one or two more levels of subfolders that group photos into shoots (for paid work) or styles (for my personal work). I also use the keywords, which is not too different than the iPhoto keywords system. I've begun using Collections, though not as extensively as some people.
With iPhoto, there are at least three copies of each image - the original, the modified, and the thumbnail. That not only bloats the size of the library, it makes it harder to find the file of the image you want.
Lightroom just keeps the original file, and never actually changes the file. It just keeps a lit of all the edits you've done, and only applies those edits when you view or export the image. The original is never changed. (this is called non-destructive editing).
Lightroom also integrates pretty seamlessly with Photoshop. If there's an edit that you can't accomplish in Lightroom, such as editing Uncle Bill out of the family photo (nobody liked him anyway), you simply right-click the photo then click "Edit in Photoshop." When you're done, save your work and quit Photoshop, and the file will show up automagically in your Lightroom library, right next to the original.