The biggest differences in the applications I've seen are in the security implementation and the Web Services implementation. Both changed significantly from WL6.x to WL7.
The new security implementation offers a lot of additional features, but the learning curve can be pretty steep, and they no longer ship a RDBMS realm. You can run security in "compatibility" mode in WL7 to use a WL6.x realm, but it's kind of a pain. Generally, if you use the RDBMS realm or have rolled your own, you'll want to update it to the new WL7 API, which also involves a fairly painful packaging process.
The web services support has also improved in WL7, though it'll have to change again so support J2EE 1.4 in a future release. You can write web services either with their graphical Workshop tool, or a series of
Ant commands the integrates with your normal build.
In a shameless fit of self-promotion, both of these topics are covered in more detail in the book that's being given away this week, including sample code for the WL7 equivalent of an RDBMS realm. Sorry, I'll stop that now.
Book & Giveaway Details
