After over 200 days of work and more than 500 commits, it is time for me to announce that the first 1.13 versions of CraftBukkit/Spigot/BungeeCord are now available to the public. Before continuing further I must stress the point that these are not release builds. They are not designed to be used on live servers, and if you run them without creating backups you will lose data. They are being released now to 1) allow plugin developers to begin porting their plugins, and 2) allow community members to contribute the finishing touches and improvements.
1.13 is different from any previous versions in several key aspects. As evidenced by some of the numbers above, it is the largest release of Minecraft ever, with a huge amount of changes, features, and bugs introduced. It is difficult to put an exact assessment on the scope of the changes, but a jump from 1.8 to 1.12 all at once is a fair approximation (i.e. all 4 previous updates merged together). In addition to all of these changes made to Vanilla, the Bukkit API has also had to adapt. The centerpiece of this release is that the Material API has been completely overhauled to both mirror Vanilla more closely, and also add BlockData features which have been missing for a long time. The flip side of this is that the scope of the changes means forwards/backwards compatibility is not assured.