6. Other Enemies
This one is up for debate as to whether that final boss fight needs other enemies Some may argue that it should be 'mano-e-mano' against that ass-hole who's been screwing up your plans for the entire game, others may say you need his minions there as a side-challenge. Personally, I think a mix of the two is the way to go. Having those extra targets really livens up the game-play, but in the end, it all has to come down to you against Darth Vader, The Don, Giant Lizard or whatever you're facing (although that would be one awesome battle between those three).
5. Length
Another key factor. Games often have a habit of making these battles almost as long as the rest of the game (Final Fantasy XI is a perfect example) and no one wants to spend that amount of time on one boss. This ties in with difficulty as the harder fights either mean it takes too many hits to cause real damage or you have to re-try it as you keep dying. A two-minute brawl just won't have the satisfaction upon victory, whereas twenty minutes begins to take its toll. Around ten minutes is golden territory.