"Here's a fun fact: The vast majority of high level characters are on accounts that were created way way before free to play launched. Those bad PUGS at high level? Same people you were playing with before September 2009."- DDO Developer Tarrant
DDO is often held up as the pinnacle of what can be accomplished with a transition to a free to play model. New players came to the game, bringing more revenue. The game retains a very high ratio of players who are actually spending money on the game, even though access to the world is free. However, it appears that us new free players (the highest character on my five-month-old account is level 6 out of 20) are not reaching the level cap.
Some of this makes sense based on the game's payment model. DDO (and soon LOTRO) are called free to play, but effectively charge players per content (whether you pay via a monthly sub or one time quest/adventure pack purchases). This is a difference from traditional free to pay models (see Runes of Magic) that don't charge for content, and it means that there are some financial barriers between new players and max level.
The long term challenge, though, is what you do if indeed your population begins to skew more and more heavily towards the low levels over time. You're not going to stop developing high level content that you were working on before you went F2P (e.g. LOTRO's Enedwaith zone, high level content in F2P DDO's early updates), but knowing that high levels make up a smaller and smaller proportion of your playerbase is bound to affect the direction of future content.