In the comments on my last post, Zizlak states that Warhammer's loot system in keeps does not harm the overall experience; if you don't win an item outright, you can eventually get the same item via tokens from killing enemy players. My gentle prodding at failing to land a loot bag after placing second in a particular keep siege aside, I agree with his point. Unfortunately, this is a problem for repeatable content incentives.
MMORPG's rely on in-game rewards to motivate players to spend their time on repeatable content. This works when the reward at the end of the line is genuinely rare and valuable. It stops working as the rewards become readily available (e.g. you already have dozens on mounts and minipets) or easily replaceable (e.g. you are still leveling, or every patch includes a gear reset). When that happens, rewards cease to be an effective incentive for doing repeatable content that is not enjoyable on its own merits.
In a more traditional group game, your team-mates provide some of the entertainment that makes each evening's gaming experience different from the previous one. Everyone who raids has some quip about the raid leader or some anecdote of a mistake that is more amusing than annoying in hindsight.
By contrast, the solo player has only the content itself, generally with a fair dose of AFK-travel time and random number generator mayhem, to keep them entertained as they repeat the same content daily. This is fine as an activity you do while waking up in the morning, or as you look for a group, but falls short if it becomes the sole point of your day's gaming.
In some ways, Warhammer and WoW's efforts to place players in groups with people they do not know might actually be doing those players a disservice; technically they're in a group, spending time doing group content, but without the camaraderie that accompanies actual friendship.