What words were omitted here?

There is nothing really missing in the sample sentence:

Discussions with the delegate are going better than planned.

In comparatives like this, if the verb can be matched to an existing way of expressing something, you needn't use a pronoun and verb.

The pattern works like this (these are just examples).

The man is richer than expected.

One can expect someone to be rich.

The discussion is going better than planned.

One can plan a discussion.

The car was bigger than expected. [same as above]

The comic book was funnier than imagined.

One can imagine something

If a verb phrase is underlying the sentence, you needn't supply a pronoun and verb. That said, all of them can take a pronoun or name plus a verb.

The dog was fatter than [I] expected. The comic book was funnier than [we] imagined.

Also, the tenses can change too if you use a verb phrase in these comparatives.

• The dog was fatter than we had imagined.
• The dog was fatter than we would have imagined.
• The dog was fatter than we imagined.