Ryzom is set on Atys--a living planet, effectively a planet-shaped tree. It's inhabited by "Homins," or humanoid races, of various sorts, and populated with many species of both animals and other sapient, nonplayer races. There is magic, and a variety of terrains and places to explore. As is fairly usual in the genre, characters take on various tasks to become more powerful; success allows them to take on other tasks, and so on.
Ryzom has a wide range of activities to engage in. There are the usual quests, which lead to new parts of the world, offer item rewards, and/or reveal some of the lore. There is consensual player-vs.-player action, from personal duels to open PVP areas and guild control of specific locations, but the main focus of the game is PVE (player-vs.-environment) questing and role-playing.
Several aspects of Ryzom make it unusual, perhaps even unique. To me, the most interesting of these is the skill system. Ryzom has an "improve through use" skill model, which is less common than the "level up" model, but not that rare. What is unique is that as you gain points in the various skill trees (fighting, magic, crafting, and harvesting), you can create unique actions by combining aspects of a skill together. For example, you can create a powerful attack by combining "Accuracy" and "Extra Damage"--but such an attack has a high cost, which must be balanced with some countermeasures, such as a higher endurance cost. This kind of flexibility is rarely seen in MMOs. You can, and should, spend your hard-earned skill points on basics like raising your hit points or strength. A mix of buying new abilities so you can form new actions and keeping your core attributes high is probably the best bet.
Crafting in Ryzom is also very flexible. You can make items from a stunning array of things; just about every tooth, eyeball, skin fragment, and twig you find might be useful. However, not everything is equally good. When crafting even a simple pair of boots, you choose materials for four slots; each material you add changes the boot's statistics, raising some and lowering others. Thus, there's a strong impetus to seek out the right ingredients for the job, and to experiment to find ideal combinations.
Another aspect that deserves mention is Ryzom's well-developed artificial intelligence. The many animal species do not stand still waiting to be slaughtered (well, usually; the first critters you encounter are pretty willing to let you kill them). They have behaviors and hunting/feeding patterns. One of my first lessons in this was seeing how rapidly I was swarmed trying to kill some predatory animals; I learned to scout the edges of the area, looking for lone stragglers with no friends to call for.
Despite the low polygon count and simple (by 2012 standards) textures, the animations are fluid and evocative.
An important aspect of Ryzom is the scenario editor, which allows players to build their own zones, quests, and so on, with other players able to enter them by invitation. They are not part of the game world itself, but spawned instances.
Ryzom's downsides are mostly to be expected given the game's age and limited commercial appeal. The graphics, while not atrocious, are clearly dated by modern standards. They're evocative and well-done, but designed for much older systems. The interface can be very busy, and doesn't scale too well to larger modern monitors. The worst thing, for me, was the lack of documentation. Ryzom's website has PDF copies of the manual that shipped with the game in 2004, which is very outdated, and an extremely sparse wiki. While the game has pop-up hints on various things, the lack of good documentation on each of the action types and other features is annoying.
Given how small and obscure the game is, I found the starting zone to be lively, and most players were helpful and friendly. The "trial" allows you to train skills up to 125, out of a cap of 250. Given the wide range of skills you can raise, you will play a long time before maxing them all out, and it's tautological that if you like the game enough to do that, it's going to be worth paying for. There's a strong sense of community and player involvement, aided by the fact that the game's engine has been open-sourced. With no initial investment (except time) required, Ryzom is worth checking out for those who want something a bit different.
Note: The Download button takes you to the vendor's site, where you can download the latest version of the software appropriate to your system.