Neo Office, OpenOffice, Zoho Docs, and Google Docs all over free to nearly free solutions for Excel, Word and Powerpoint on Lion. You don't need to downgrade to Snow Leopard to achieve compatibility for those formats.
If you dependended on Entourage for your mail, and used it for POP or local storage of Exchange mail, you have one of two options, Upgrade to Office 2011, or install 10.6 on an external hard drive and connect to a Mac that supports it. In the 10.6 install solution, you then move your Users -> yourname -> Documents -> Microsoft User data over and install your older version of Office on that drive. From there you can export it to more compatible formats as outlined here*:
Sadly color labels while supported in Lion, are no longer sortable in Lion. This is a known bug in Mac OS X Mail. Hopefully some other Mail program will support that kind of categorization soon, if not already. A lot of people like Thunderbird for Mail, though I haven't tried it in ages. But once it is compatible with Mail, it is easy enough to import it into Thunderbird. You can use Mail while booted back into Lion to reimport the Mail that got stuck in Entourage.
Quicken by the way is now Lion compatible.
If there is something else that isn't Lion compatible, check for alternatives here. If you still can't find an alternative for a specific application, don't be afraid to ask.