For the uninitiated, raw-workflow software supplies tools for those who don't require the compositing and effects capabilities of Photoshop, delivering a more targeted, streamlined approach to photographic production tasks--specifically viewing, selecting, organizing, retouching, and outputting photos. (For an example of what that means, see the first few paragraphs of the Lightroom review, linked above.)
Aperture's file handling ranks as the most important change introduced with the 1.5 update. One of its biggest weaknesses had been the way it secreted all your images from view into an operating system package file, the type usually reserved for installation programs. Now when you import your photos from removable media, it copies them to wherever you want; it also allows you to reference files on optical discs or detached drives. And you can still create a replica of the Library, called a Vault, for backup purposes.
However, the program remains fairly restrictive about the Master--original--files. For instance, you can't rename them. When you import the files, Aperture builds its database of references and gives you powerful tools for automatically naming the Version, which is the referenced Master that you work on. When you export the images, you can use the same powerful tools to name the final file. However, Apple's philosophy, and the philosophy of many Aperture acolytes, is that you have no reason to care what the file is named or where it's located. I disagree with this for a variety of reasons, but I'll offer what I consider the most compelling: many of us don't run Aperture (or Lightroom, or Photoshop) all day. We sometimes have to close applications. Who wants to launch Aperture simply to locate and copy a file? Ironically, the touted integration with Apple's iLive '06 and iWork '06 software suites was likely necessary because of the way Aperture's file handling forces you to bypass the Finder. And personally, I'd like more flexibility in renaming files from both Aperture and Lightroom, such as the ability to automatically rename using the file's keywords.
Furthermore, this closed-system approach affects Aperture's Photoshop integration to its minor detriment. Rather than storing adjustments with the original file in an XMP sidecar, Aperture creates the history and metadata XMP file only when you export. At the very least, an Edit With... option could make it a single-step process. Version 1.5 adds an Export API plug-in architecture, however, which would allow third-party software developers to greatly strengthen the ties between Aperture and Photoshop, as well as with other apps.
Aperture does maintain one significant workflow advantage over Lightroom: a Photoshop-like Proof Preview setting, which Lightroom lacks, allows you to work in a more color-WYSIWYG environment.