1. Remember you don't have to use iPhoto. It's not compulsory. You can have your photos in folders on the HD like you always did. It's much more limited, doesn't offer non-destructive editing and so on, but it is what you're used to.
2. iPhoto works on a Library Principle. Like iTunes. Every song is in the Library. Each playlist contains links back to the Library. So a song can be in any number of playlists and use no extra disk space. Same with iPhoto. Every photo is in the Library. Each ALbum contains links back to the Library. So a photo can be in any number of Albums and use no extra disk space.
Consider the followng example: A snap of two kids and a dog on a beach. How do you file that? 'Summer 1999', or by Kid A or Kid B or Dog? Or even beach?
How about all of them. Just think, if it's a favourite photo you can have an album of those drawn from all over the Library - your 100 favourit photos, while they are still also in Albums for the people in them, the places, the event and so on.
How to find the photos not in an Album:
FIle -> New Smart Album
Album -> is not -> Any