$100 is the online game equivalent to average supercar pricing. 
As someone who has bought DLC before, I've seen a lot of stupid things publishers have done to make a quick buck... and I buy it anyway because I loved the game and wanted the additional content, such as when I bought Jill and Shuma-Gorath in Marvel vs. Capcom 3 even though they were already on the disc to begin with. Nevertheless, this blog post is going to be about the mistakes publishers make when it comes to releasing downloadable content (mostly in games I've experienced), and how they can be mitigated, if not fixed entirely.
Mistake #1: On-Disc DLC
This is probably my biggest complaint when it comes to downloadable content. Remember back in the days of Marvel vs. Capcom: Clash of Super Heroes when you could unlock a character simply by playing the game a few times? Well, since then, Capcom has (for whatever reason) decided to change business and development practices, and come the release of Street Fighter x Tekken, if someone wanted twelve extra characters, he would have had to spend out another $20 (plus however much he already spent for the game) a little over two months later in order to get them.  Even though they are technically already on the disc that the customer paid $50 for (if he bought it on release day), that is a total of $70 being spent on a game, which is slightly more than average retail price.
Fun Fact: Some gamers got to play all these characters early through hacking.
And that's just for the characters. If you wanted alternate costumes for these characters, you had to pay another $41 for those.  So instead of paying $50 for a disc full of content in Street Fighter x Tekken, the total price comes down to $101. Which is rather pricey for a crossover fighting game, when you think about it.
Take another big fighting game that came out the same year: Tekken Tag Tournament 2. Like Street Fighter x Tekken, the game had characters, stages, and customization parts that, even though they were on-disc, were locked. Unlike SFxT, all of this additional content was gradually unlocked with title updates for the game (with the exception of the content that were pre-order bonuses, in which case that content was unlocked for the people who hadn't already pre-ordered the game and weren't already using that content).
Everyone in the top row only costed two months of waiting. Two. Painful. Months.
Alternatively, and this especially goes for fighting games, go back to the original formula of unlocking characters as the game progressed. I am pretty sure that the only fighters to have done that recently were BlazBlue and SoulCalibur V. By playing the game to get new characters, the players can get that sense of reward that always came whenever they did so in older games, and we all want to see happy gamers.
Mistake #2: Time-Limited
Thankfully, it's a really rare concept, and in most cases it doesn't really provide anything major to the game itself when used. There are always exceptions to the rule, the biggest one being Activision's Marvel Ultimate Alliance 2. For a short time, several additional characters were released as DLC, and then they just disappeared. They then briefly came back, only for them to be taken off the marketplace yet again.  As of this writing, there has not been another release of the Marvel Ultimate Alliance 2 character packs.
What's a guy gotta do to play as Carnage, huh?
The whole idea of time-limited DLC seems kinda pointless, since not only can some potential goodies get locked away forever with no chance of ever getting to use them, but those who do have them can basically use them as bragging trophies, which is not a cool thing to do to begin with.
As far as I'm aware, with the exception of a few Nintendo DS games, there haven't been many cases of this type of DLC at all, and especially any after Ultimate Alliance 2, so in a way this problem is already more-or-less fixed. If it weren't, the advice I would give is simply not to do it at all. Removing the content from the marketplace after a time was probably a bad idea, since it gave people less incentive to play the game later, especially if the missing DLC has a fan favorite character (e.g. the Juggernaut) that would draw interest to the game itself.
"You can't leave me out of this game! I'M THE JUGGERNAUT, *%#^@!"
Mistake #3: Ending DLC
If a video game has a well-written story, then it would obviously have a solid beginning and an equally solid conclusion. It it just a simple fact of storytelling, nothing more. So when developers decide to release a DLC that is for the "true" ending of a game that players have seen the ending to, it makes the original ending less impactful than it was originally intended to be. This is exactly what Capcom did with Asura's Wrath, case in point being the DLC for "Part IV: Nirvana", which was released just a little over two months after the game's initial release and presented itself as the true ending of the game... which players had to pay if they wanted to see. 
Needless to say, Asura isn't happy about this.
If it were a book or a movie, it would definitely be looked down upon if readers or filmgoers had to pay extra to see its ending. So why can't the same be applied to video games? Even though it a mostly online medium now, it doesn't mean that parts of a game's story, especially the ending, should be viewed a something separate as the rest of the story.
The best solution for ending DLC? Include the proper ending into the finished game. There is really no other solution I can suggest for this type of DLC. As someone who used to write stories regularly, as well as someone who's played a few games that could be considered unfinished, I can understand that sometimes circumstances may force something to be left incomplete. However, if that should arise as an issue in development, then it should be left to the developers and the publishers to try to work out an arrangement in order to release a finished project.
Other good ways to promote DLC
Season Passes are a great way help promote DLC for a video game. While it may not include everything DLC-wise the game has to offer, they do offer a sizable portion of the larger content for a slightly lower price than all of them separately. A lot of recent games have Season Passes for some of its DLC, such as Borderlands 2, Mortal Kombat, Injustice: Gods Among Us, and Bioshock: Infinite. For example, Borderlands 2's Season Pass includes four campaigns plus a new difficulty and a level cap inrease, which are the most notable of the Borderlands 2 DLC offerings at the present time.
Game of the Year Editions
Sometimes, when a game does exceptionally well in the eyes of critics, it sometimes gets the honor of being called "Game of the Year". In response, publishers will release the game again at a lower price point and labeled as "Game of the Year Edition". In many cases, such as Batman: Arkham City - Game of the Year Edition, the game included all the DLC that was released prior, including the relatively recent "Harley Quinn's Revenge" DLC campaign.
Is downloadable content a bad thing? Not at all! When you think about it, DLC has been a part of our gaming lives for a considerable amount of time, even if it went by unnoticed. Whether it was the GameLine for the Atari 2600, or some armor for a noble steed, both good and bad DLC was a part of it. Hopefully publishers who put out bad DLC will learn from their mistakes and do something right... but until that time, we can at least point out those flaws.
Now go forth and pretend this armor isn't useless.