What can't I do with Google Spreadsheet that I can do with Microsoft Excel?

In my opinion no; however your needs are really what's important here.

What are you trying to accomplish?
One of the best reasons to use Google is if you need to share a workbook with other so that you can all edit the document at he same time.
As Wayne points out, Google also has a range of scripts, graph types, web functions, and other doodads that Excel doesn't.
That said, you should also check out Microsoft's Excel Web App, on their Live.com / skydrive / 365 platform. I tend to prefer it to Google Spreadsheets, but again, it depends on what you are trying to achieve.

I suggest that it is a question of value and effectiveness. I started using the full suite of Google programs about three years ago, and generally find that they can do everything I need them to do, and after a little bit of experience are easier to learn than some of the newer versions of the Microsoft products.

In an office environment, especially one which is new, or doesn't have a large installed base and institutional knowledge base on the Microsoft products, going with the Google suite of products might make the most sense, especially given the cloud based sharing mentioned above.

That being said, I still have at least one copy of Excel which I use for more complicated projects, because in my case, I have more experience with it, and I don't see the value in learning a new "language" especially since once a program is developed, I can generally convert it to Google Spreadsheet anyway, once it is "in production".

I would not recommend Google program since they are years behind in the functionality you need to develop models and share them. In all honesty, don't even waste your time.

I find that in most cases these days a Google spreadsheet will do what an Excel spreadsheet will do. Google has made significant improvements in the past year or so in their app. I think is is actually better for collaboration than Excel. If I do anything out of the ordinary I still tend to go to Excel, but I think has more to do with 20+ years history using it. I am old enough to remember having to force myself to switch and use Excel, because Lotus 123 was so comfortable., but I could also see back then that Excel was the product of the future. I think Google is the product of the future today.

Google is a good tool depending on your Excel expertise.

Whenever I am asked to score myself 1-10 on excel proficiency (common question in interviews these days), I say that I am a 7-8, but will run circles around most folks who think they are 10's. In my last 3 companies I have worked for I have been the go-to guy for Excel questions, and one of them was an IT company.

The more you learn about Excel, the more you realize you don't know! Once you have become proficient in macros, lookups, embedded if's, etc., you can start to really unlock the power that is there. Excel also interfaces so well with so many other software options.

Google is a good interim for collaboration, but most documents need to start in Excel first.

Oh man, where do I begin. Formatting, shortcuts, version control, macros, user experience.

That said, I use Google Docs to quickly share ideas. Never for formal presentations or even big decisions. Better to have something that looks good and functions.

You can't access it if you're not connected to the internet or lost someone else's permission to access it (if it's not your document) and haven't already downloaded it.

The main advantage of excel compared to Google Spreadsheeta is it is highly secure. The data which we enter into the excel sheet is completely secure with us.

The another important advantage is you can use it offline, there is no need of internet connection and there will be no interruption in our work due to network issues.

The options in Microsoft excel is good for all the users from beginners to experts.

The manipulation and loading speed will be fast if you have more data in the sheet.