The biggest issue with Office Web Apps isn't what's here, but what's missing and will be missing at launch: the ability to synchronize your work in Office so that, wherever you are, you always have the latest versions of your documents.
For all of its look-and-feel mimicry of Microsoft Office, at heart the Web-based and the client-based versions of the software are surprisingly separate from one another. This lack of integration and synchronization will most likely lead to a great deal of confusion about where the latest version of a document is located.
The problem is that there is no automatic synchronization between your work on your local PC and your work in Office Web Apps. Let's say you create a document in Microsoft Office, and want to work on it later when you're away from your machine. Before you leave, you'll have to remember to upload the document to Office Web Apps; if you forget, it won't be available to you.
After you've worked on it online, when you return to your main machine you'll have to remember to download the document to work on it. If you forget and work on it locally, you'll be working on a local version which differs from the latest version, which is stored on the Web.
Making matters worse is that when you upload a file from a PC to Office Web Apps, you're not given a warning if you upload a file that will overwrite the file that's already there. So it's very easy to overwrite newer work with older work.
This is a very significant shortcoming, and if it remains this way it launch, it will be at least one way in which Office Web Apps will be inferior to Google Docs. In Google Docs, this integration between online and offline is built in; you're always working on the latest version of your document, no matter where you are. Synchronization is built in -- assuming you've installed the Google Gears app, which saves documents to your hard drive. (On the other hand, Google Docs has its own shortcomings compared to Office Web Apps, such as an inferior interface and an inability to create documents as sophisticated-looking as those created in Office. )
What makes this deficiency all the more surprising is that Microsoft already has excellent synchronization technology that to a great extent solves this problem. Windows Live Sync automatically synchronizes files and folders among multiple PCs. If Microsoft were to link that synchronization to Office Web Apps, it could keep files in sync between your local hard disk and your Web-based SkyDrive store of information. However, at this point at least, that is not in the works.
Finally, sharing documents with others is awkward at best. You can't share individual documents. Instead, you have to share entire folders -- every document in any given folder is either shared with others or not.
The way you share is confusing as well, because it requires a multi-step process in which you can become easily lost. This is because it uses the SkyDrive interface for managing and sharing files. Microsoft would do well to change this before final release.
The bottom line
At this point, Office Web Apps is not much more than a glimmer of what the final version will be at launch. Based on what I saw, though, it will be a useful complement to the client-based version of Office. Its ability to display Office documents in all their fidelity is particularly noteworthy, as is the ability to work remotely on the same document with others. However, at this point it is not clear what features will be present or missing at launch, so it is too early to tell how powerful Office Web Apps will be when creating documents.
Despite that, it's already clear that the Web-based version of Office will find widespread use by almost everyone who now uses Office, not just for the Web-editing features but because it can be used to share documents with others.
However, the lack of automated synchronization of files between local computers and the Web is a very serious shortcoming. Because of that, Office Web Apps will not be as useful as Google Apps for those who often work on documents from different locations.