As a lot of people know, apex.oracle.com is the customer evaluation instance of Oracle Application Express (APEX). It's a place where anyone on the planet can sign up for a workspace and "kick the tires" of APEX. After a brief signup process, in a matter of minutes you have access to a slice of an Oracle Database, Oracle REST Data Services, and Oracle Application Express, all easily accessed through your Web browser.
apex.oracle.com has been running Oracle Database 12c for a while now. But a lot of the 12c-specific developer features weren't available, simply because the database initialization parameter COMPATIBLE wasn't set to 126.96.36.199.0 or higher. If you've ever tried to use one of these features in SQL on apex.oracle.com, you may have run into the dreaded ORA-00406. But as of today (January 30, 2016), that's changed. You can now make full use of the 12c specific features on apex.oracle.com. Even if you don't care about APEX, you can still sign up on apex.oracle.com and kick the tires of Oracle Database 12c.
What are some things you can do now on apex.oracle.com? You can use IDENTITY columns. You can generate a default value from a sequence. You can specify a default value for explicit NULL columns. And much more.
You might wonder what's taken so long, and let's just say that sometimes it takes a while to move a change like this through the machinery that is Oracle.
P.S. I've made the request to update MAX_STRING_SIZE to EXTENDED, so you can define column datatypes up to VARCHAR2(32767). Until this is implemented, you're limited to VARCHAR2(4000).