Rob, I was trying to stay clear of this question, but many of the major advantages have not been mentioned. Some of these have been touched on in broad strokes, but deserve more attention. I will mention a few that have been mentioned for the sake of completeness.
1) Actual Roleplaying Potential. The term roleplay existed before it was coopted by the hobby, and the idea of getting into the skin of a character, the 1st person narrative mode, is nearly impossible when done through the translation of the keyboard. A player can write or respond in the first person, but this is a mental approximation. Most good GMs with talented roleplayers experience the phenomenon of players falling into character to some degree; and indeed, this could be called one of the major goals in playing a "RolePlaying Game'.
CRPGs are always going to have a huge disadvantage in this.
Similarly, many facets of the game are made enjoyable live that are very difficult to replicate through the keyboard. Buying goods, interacting with town folk, etc, is something a good GM includes in the Roleplaying, and this is much more 2-dimensional in a CRPG.
Acting is, by the way, a skill. People who are good at it excel in live Roleplay, as they often have that ability to get into character, at least to some degree.
For all of these real 'roleplaying' effects, CRPGs are a mere pale approximation.
2) Variable, Customizable Interface. The continual feedback loop of players and GMs creates better games for both, short-term and long-term.
Houserules, player style preference, and direction of the campaign are all trailmarkers watched closely by the experienced GM. A good GM can see when a rule needs to be tweaked to work better with that particular group. Similarly, when the Players spend more time working with the politics in a setting or get in-depth with the local mercantile situation, the experienced GM automatically mirrors this and includes more of this in the game, probably adding hooks and possibly whole backgrounds to enhance the part of the game the players enjoy.
Long-term, this feedback loop becomes more critical and personal. Campaign style games are what most rulesets are actually written expressly for, with the idea of character growth, personally and within that setting, as design goals. The experienced GM can shift and alter the games and campaigns to personalize the long-term campaign, the adventures, their goals, the trials, and creating a culmination point based on the game needs of those players and that group. Despite the improvements in online gametables, etc, they still cannot match up to responding to the body language and exact needs of the players.
3) Synergistic Social Interaction. A few people have glossed over the social dimension. But I add the synergistic part as real social interaction often involves increasing real friendships, adding to the real social lives of the people involved. This is nice in a gaming community, but it is once again one of those places that the long-term, live campaign outstrips any online approximation.
Gaming with other gamers is fun; being around people that you only see gaming. But as with many real-life social interactions, the synergistic effect comes into play when a person builds real, long-lasting friendships through their groups.
I've kept some players since 5th grade, and a number from high school. We all see each other socially a little bit, but it is our gaming that keeps us able to see each other every three weeks. And these relationships are 25-30+ years long.
And as one gets older, the social aspects grow. Most of my sessions include people making dinner, spouses coming along and sharing dinner (and advising...always funny), live props (they suited me up in Lamellar a few months ago over an adjudication), and not to mention the characterizational comment, "At one of our WineTastings, a gaming session broke out', as we normally go through a fair amount during every session.
I do run some games online, but they don't really match up to the social interaction level here...
OK, back to work. Darn you for pulling me away with a question that needed some cogitation.