A happy gaming group. When a campaign ends and the main characters finally reach the end of their journey, literally or figuratively, the success of the game wholly depends on how fun it was for all the players and the GM.
Some think what matters is whether or not lose ends, plot points or narrative threads were all properly concluded or tied up, but I don't believe those are even required. Far too many stories that I have enjoyed have many unanswered questions, or leave certain threads unfinished. But the joy of a successful campaign is not tied to literary perfection, in my opinion.
In the end, table fun is king. If your group ended the campaign and you all felt it went well, then it did. No one else outside that group has the right to say otherwise.