Jonathon used the example of Call of Duty 2 to demonstrate his argument against balance. Jon thinks that in CoD2, the game is fundamentally the same throughout; enemies just get tougher and greater in number. The experience is always the same and this is the reason that so many people get bored and fail to finish the game. Although I haven't played CoD2 beyond the demo, I have played the original Call of Duty (with United Offensive expansion pack) on the Mac. I found some areas of that game intensely hard, whilst others were relatively easy; for me Call of Duty exhibited many of the characteristics that an "unbalanced" (and, Jon thinks, a therefore more entertaining) game is supposed to exhibit. For example, the sniper section of the game is several times more difficult than many later areas of the game. It also offers a completely varied gameplay style from other sections of the game. Here you're expected to get as close to the sniper as you can without getting shot, but in other bits of the game you're the one that's sniping, or you have to engage in tank battles. These examples don't go along with Jon's suggestion that CoD is a game that slowly increases in difficulty and offers very little varied gameplay.
I have a feeling that Jon is confusing balance with storyline. CoD has a very linear story that progresses in a plodding fashion as the game goes on -- I assume that he became bored with the Call of Duty 2 storyline, which he may believe didn't offer enough of a personal motivation for him to continue playing the game. In any case, Second World War shooters are very subjective games. I know several people who will not play them on principle, simply because they're "just like the last one". But to brush aside the Call of Duty series for being too balanced because of a lack of varied gameplay styles is a poor argument.
However, the real reason that Jon's argument falters is because balance is a strong feature of many retro games. Take Space Invaders or any other arcade game with a high score (Geometry Wars is a great example). These types of games rely entirely upon balance for the game to work. Your ship's capabilities and the type of enemies never change, but the difficulty increases slowly over time as you continue to beat back the enemies. The whole gameplay mechanic is about keeping everything the same, but making it harder. A vast swathe of games are made this way, and an enormous number of people enjoy and become addicted to so called "balanced games". It's possible that this style of gameplay has been extended too far, but to bin this style outright, taking the high score and most arcade games along with it, would be a great shame.