Example 1: commands in general
1: In chat, type "/gaMeRu"
2: press TAB
3: check that it will complete to /gaMeRule instead of /gamerule (it used to do that in older versions)

Example 2: block coordinates
1: face a close block (have the outline on it)
2: in F3, check that there is a "Looking at" tab on the bottom left that displays that block's coordinates.
3: In chat, type "/setblock " (with the space at the end)
4: Press TAB
5: check that it does not complete the command with the "Looking at" (mentioned above) coordinates as it used to in previous versions.

Example 3: effects
1: In chat, type "/effect give @p " (with the space at the end)
2: press TAB
3: notice that nothing happens
4: Now, try with "/effect give @p spee"
5: press TAB
6: notice that nothing happens, even when there is only 1 possible completation (speed).

The same happens with other commands