Casual players generally have time to invest in one character in one game. We don't really have the time to learn all of the mechanics and subtleties of one class/race/talent system, much less operate a stable of variant characters and learn different interfaces and systems in different games. It is a big investment of available time for a casual player to learn to play a single kind of player in one game.
Also, the casual player isn't going to be able to participate in so-called end-game raids or be a significant part of any Uber-guild - that's just the nature of the the casual beast. In games where end-game character advancement is defined by the employment of hardcore grouping/raiding to acquire advanced gear, the casual player's character advancement generally just ends upon reaching the level cap. From there on, it's usually just a matter of buying stuff from other players to improve your character, because you cannot even journey to the areas necessary for any high-level content.
Thus, the casual player is faced with a rather bleak choice (at least as far as current games are concerned); start a new character or just do the same things over and over to earn game money with your current one without any prospect for significant character advancement.
The question is - why do developers do this? Why force level caps and talent/skill tree limitations on characters when there could as easily be limitless andvancement based on diminishing returns and a real-time curve that would prevent people from burning through advancement to become overpowered in terms of end-game content? Developers could build in time constraints that would provide them with the time to generate suitable higher-end content if any significant number of players seriously push the power envelope. Besides, if after 10 years of character development one has a super-powerful character, so what? Isn't that a good reward for 10 years of player commitment?
For example, let's say that I decide to advance my character in fire spell potency. Why not make it so I can increase the damage of all my fire spells endlessly, if I choose to sacrifice all other areas of advancement? If there is a system of diminishing returns and increased time investments, it could work and also provide incentive for long-term investment in the game regardless of how hardcore one is. By slowly decreasing the increased potency (+5 damage per 1 hour invested, +5 per 5 hours invested, +5 per 10 hours, +4/12, +4/15, +3/15, +3/24, +2/24, +2/48, etc.) over time invested, one could eventually settle on a minimum of, say, +1 damage increase per 5 days training that a player can invest in at the expense of all other advancement ad infinitum.
This means that they could, even as casual players, carry on the advancement and customization of their character forever. Players could endlessly customize/advance their character. They could focus on one or several traits/talents and increase them as they see fit over time. Highly potent combinations could be penalty adjusted via a cross-training system that might increase the time-investment necessary (to, say, cross train in both fire spell potency and dual sword wielding, or Health (hit points) and Agility, or to decrease the rate of gain; other combinations might lend themselves to natural increases in trait or skill gain speed.
Although such training combinations can be adjusted for speed in advancement, there would still be no limit to what you can do. It would just take longer to increase some combinations than others.
Going down set, constrained paths with structured caps is just so ... 15 minutes ago, man.