Well, "Good" roleplaying, in my opinion, should be something that utilizes the full capabilities of the very core of the idea while maintaining high interest.
Core ideas of Roleplaying?
Start with the first, it's an Interactive Story. As that is simple it's also the hardest to keep in mind for many people on Gaia. And I do slog through a lot of Gaia. On one hand you have people that forget the "Interactive" part and are more focused on just crafting their own personal storyline. They resent other people interfering with what they are doing, already have the plot figured out, and anything that interferes with it will be ignored or bitched at. Those people might as well just drop the facade of Roleplaying and go write a story. The other people forget the Story part of it. Despite popular ideas out there creativity does have quite a few rules to it. Stories do have parts to them which make them stories. Three Act Structure, climaxes, resolutions. The vast majority of roleplays on Gaia miss this entirely, generally devolving into "Sit around and chat" sessions. Even Seinfeld "The show about nothing" actually had people going out and doing things.
Two, it is only Roleplaying as long as you remain in character. Again, a basic thing. But far too many just devolve into OOC chatfests with the IC forgotten or wholly secondary. When you have more OOC going on than IC is generally a bad thing. People excited and chatting out about the plot is one thing. But when it becomes just a general chat location with occasional RPing... You're not learning much.
Three, The Unwritten Rules. Roleplaying is a game, games have rules. As due to the nature of Roleplaying itself the rules thend to be fairly fluid. It's hard to "Learn" good roleplaying because the rules vary from place to place, roleplay to roleplay. Gaia however has a lot of "Unwritten Rules" in its Roleplaying community (And hell, most communities do). These rules are often of a type that I consider detrimental towards Good Roleplaying in general. But this is due to the nature of the community being a core of anime fans and teenagers.
Four, Fluidity and Movement. Roleplaying is by far a very fluid means of storytelling. You have anywhere from 2-10 people or more clustered around, all with different goals, different ideas. There is no real definition of what is winning, losing, or even really correct. Roleplaying can take a story in directions that no one at the table ever imagined was possible. And due to its fluid nature it needs to keep moving or it will become stagnant and die. Gaia is pretty poor on both of these points. Roleplays stagnate and die by the hundreds there, and most roleplayers lack an acceptance of Fluid Motion in their story telling do to having their plots set in stone (See point one), or due to unwritten rules they abide by that discourages sudden shifts, changes, or unexpected events (See point three).
Gaia is a place to use, as needed. I wouldn't suggest it as a starting point however. Several reasons, as an old, and fairly populace website it's very fast moving and there isn't much in the way of Welcoming Committees or other help out there for the new player to keep them from getting overwhelmed by the pace. Once you know what you're doing however, learn the various watch words to know what you want and what you don't, you can slog through it fairly quickly, and its large, busy population means you can usually find at least one thing quasi-acceptable to your tastes whenever you go searching.
Actually the place I would recommend to learn roleplaying is the place I, and many I know, cut their teeth. Kitchen table, with books, maps, dice, figures, all the old accoutrements of Roleplaying in your grand forefather's day. It teaches you a lot of the basics in a fairly fast paced, fluid way. You can play with your friends, have a lot of fun, get out of the house and go to a new location, and pick up a few habits that are good things to have in Roleplaying and often not used on Forums. Adaptability, knowing your role, working with groups, how to blend interpersonal subplots with overarching storyline plots, knowing how to accept and move on when things you don't like happen, hell, how to take random flukes and twists of fate and turn it into excellent storyline fodder.
Part of what I don't like about Gaia. The Unwritten Rule of Plot Armor that runs rampant on that site. Nothing happens to anyone unless they already decided months ago they wanted it to happen.
Metaphor time. Roleplaying on Gaia, at its best, is like looking upon a carefully tended and sculpted English Garden. Artificial, cold, not a single leaf out of its perfect symmetry or guided plot, making a landscape of perfect geometry and precision planning. Roleplaying on say, the tabletop, is like stumbling across a primordial evergreen forest. Vast, ordered by with anomalies that you would never see in the Garden. Formed by primal forces out of the control of Man, making something that cannot be imitated by Man.