Maybe it's just me, but I consider DDO clearly a game to play, cancel when you run out of content you'd like to explore, and then resubscribe every few months to play the new stuff kinda game.
I'll buck the reviewer trend by saying that DDO does have a lot of content. I play 10-15 hours per week, which I'd say is substantial and somewhere near the average, and during two months of beta I only got through maybe 1/3rd of the game's content. I had a golden rule; if something wasn't fun, I wouldn't do it. DDO's content is surprisingly fun, taking the best elements of AC1 dungeon design to the next level, everything from the traps to the levers.
At some point, playing DDO you might run out of fun things to do - but the journey is the most fun of any PvE MMO I've yet played.
What would be really great, though, is if they had some sort of 'station pass' with AC1. AC1's quests are similar in style to DDO's; if you like the quests in one, you'll like the quests in the other, or so it seems.
AC1 has an unbelievable amount of content from its six years in operation; not only that, the game's levelling curve has been revamped such that you could pretty much play through all the game's quests without having to spend much time at all levelling - the "level by questing" philosophy has seeped into AC from DDO, as most quests now give XP rewards. You wouldn't level as fast as you would with straight dungeon hunting, but you'd see a lot more interesting things. In AC1's PvE, where you're not really competing against anyone, you can add a lot of challenge and spice to the game by telling yourself to use no spoiler sites (They've been good about making everything doable without) and/or using as much quest gear as possible, not tinkered loot.
Anyone who enjoys DDO but isn't sure it has enough content - this would solve their problem in spades.
Especially if Turbine launched some sort of alternate rules server for AC1, where all quests have their XP significantly increased, portals are more limited to encourage exploration, and after X number of monsters killed per day you see serious diminishing returns.
AC1 has something like 600 quests. Most span multiple dungeons, and run the gamut from lever-pulling to trap-dodging to puzzle-solving to platform-jumping. The biggest "raids" in AC1 are completeable with 8 people and do not require significant anal-retentive leadership; they're more on the forgiving side. In fact, AC is one of the few games where you genuinely have more fun if you don't MinMax.
If Turbine doesn't roll out something like this eventually, it'd be a loss to both games.