Description
This thread X/ This thread is X, is a simple meme, commonly used on forums to portray a poster's opinion of a thread, and as a seal of approval, for example:

or

Requirements for use
It is usually used when any of the following statements are true:
1.A thread has potential.
2.A thread is epic/fail.
3.A thread is lacking something and in need of something else.

Other Variations

Another variation is "This thread…" which is simply a thread started for the purpose of displaying images such as the ones above.

Origin
The meme's origin is unclear, but most likely begun on 4chan, before spreading throughout the web.