How does full/pending spot work?

It's an esoteric distinction: functionally, the clubs are the same, whether they are "pending clubs" or not. In earlier iterations of the site, there actually was more of a difference, but not now. The only difference between the two, now, is that pending clubs are shown in an extra list that full clubs aren't. In that respect, it could even be said that being a pending club is more desirable, as it's that much more likely that users will find it in random browsing.

As you said, it depends mainly on content and fans, but it also depends on channel, for instance, I created the link before the Versus and Mashup channel was created, however, once it was it was decided that there were enough versus and mashup spots to create a new channel, thus, despite only having about 20 fans and not too much content it became a full spot. I imagine that it is also easier for a spot to become full in the Fanpop Users or Versus and Mashup channel than television because they are smaller.