There are so many issues associated with purchaseable game add-on DLC, it's hard to know where to even start. DLC is not inherently bad, but there are just so many ways for publishers to abuse DLC, and for DLC to do more harm than good. Below I've listed a few of the types of DLC that are available and what some of the pros and cons for gamers are.
Full downloadable games:
DLC of this type takes the form of stuff like Xbox Live Arcade games and games you purchase through a download service like Steam. This is probably the best use of DLC that there is. Being able to download lesser known, independently-developed, or older games that aren't on store shelves anymore is a great use of DLC. Another significant bonus is that there is no need to find the DVD and put it in your drive when you want to play. For newly-released retail games, however, it is really unfair to charge the same price for the download as for the boxed retail copy of the game, since unlike the boxed retail copy of the game, the downloaded version can't be resold to recoup some of your investment after you're finished with the game. $50-$60 may be acceptable for retail boxed copies of games, but gamers should not accept the same price level for downloaded versions of the same games.
Downloadable mission-packs or map packs:
This is an OK use of DLC, as long as the pricing of the overall game experience is fair to gamers. If publishers and developers charge full price for a game, but include only a limited number of maps (e.g. Shadowrun), and then charge you extra to download the maps they left out of the initial game, then that crosses the line and the DLC feature is being used to abuse fans/customers. However, if the price for what is included in the initial game purchase is fair, then the only downside here is the same downside that has always existed with expansion packs: fragmentation of the user base. If you spend money on a map pack, but only 5% of other players do, then when you are looking for an online match, there will be far fewer matches available which use the maps you downloaded. For single-player mission-packs, fragmentation of the user base is not a factor at all.
Microtransactions for purely cosmetic in-game items:
I don't have any problem with this, since there is no impact on game balance if the purchaseable items are
purely cosmetic in nature.
Microtransactions for game-impacting items, experience, or competitive advantage of any kind:
If the game itself is
free to play, but then microtransactions are used to buy perks and better equipment, then there is not really anything to complain about (e.g. Battlefield Heroes, Second Life, Maple Story, Entropia, etc.). However, you certainly won't find me playing games like this.
However, developers and publishers who charge $60.00 for a game and then use DLC to offer game-impacting downloadable equipment/features for a fee (e.g. Battlefield: Bad Company, Hellgate London) should be boiled alive and then hung until dead.
This is true DLC abuse and should be opposed vigorously by all real gamers who value skill-based gameplay and an even playing field. In my opinion, microtransactions of this sort amount to game publishers/developers going into the business of offering paid cheats for their competitive online games. It represents complete corruption of the spirit of fair play and competition in online games.