More information than you could possibly need.
I'm not sure exactly what you mean by useful.
It's very difficult to be useful in a way that is uncommon. Currently we have a lot of people who have played for a long time, and a low population means that the demand for crafted items and special materials used in crafting is not high enough to make much competition.
There are still things that are more or less common though. But if you always compare yourself to everyone, you'll probably just discourage yourself.
Anecdote: For some reason I gave up my dream of digging in the Prime Roots and mastering all armor starting with Matis. Even though armor crafting is common, and most master armor crafters have plans for all races, I still wanted to be able to wear anything any time I wanted.
Since you've played a few MMORPGs before, you know what combat roles as important. Affliction casters and ranged combat tends to be overlooked, but they do play important roles. While nukers are critical for killing things, affliction casters can make things much more manageable by dealing with additional aggro. Ranged combat can be a devastating strategy to employ in PVP.
Basically, pick a primary skill for each of the 4 skill trees.
Your local land is ideal for digging and crafting; although materials from the Prime Roots lands are good as they're universal for the 4 races crafting plans, it's not as safe a place to dig; and if you pick a land for digging/crafting that's not your native one, do it early on so that you're not spending skill points on skills in the land you don't plan to use as your main. (although it's really only an issue in the first half or 2/3rds of your crafting)
2H is easiest for melee as it contributes to damage at lower levels, lowering the need for nukers. 1H often becomes a secondary skill they raise later that is better for tanks, as it makes them a lot tougher. Ranged REQUIRES you to dig and craft for the ammo. It's very demanding. It's useful for pulling aggro as some weapons have the potential for greater range than nuking, and other weapons have the ability to hit multiple targets like bomb nuking. Rangers are probably most useful in PVP combat.
Healing and nuking/elemental is easiest for magic. It's hard to really pick one over the other, but you probably will. Something like 2H & healing are a good two to pick, as you're not being redundant with two offensive abilities like 2H & ele, although they both do have their uses. Carrying an amp on you means you can kill from a distance, but carrying a 2H sword on you still leaves you without the protection of heavy armor. Then again when something sneaks up on you, they can interrupt your casting, while melee skills mean you can smack it, and with enough of a level advantage, possibly bare-handed.
Digging and crafting in Ryzom is the backbone of the game. With the lore of a world without minerals explaining the constant degrading of equipment and inability to repair, constant crafting is required to supply everyone. It's also the main source of income. Those who don't at least dig will find themselves barely scraping by to afford their teleporter tickets.
CRAFTING:
Armor crafting is common. EVERYONE needs armor, from casters, to even diggers, to tanks. That said, light armor is most commonly used (casters of all varieties and diggers use it), followed by heavy armor for tanks, and medium armor is probably less common, as it's more for people who either want a single all-purpose armor, or for the rare people who use ranged combat.
That said, many people will use heavy armor crafting to grind their armor levels high, as it uses more materials per item crafted, resulting in more XP. With the armor crafting skill levels gained from this, they can "overcraft"* other armors (either other weights (light/medium), or other parts within that weight (sleeves, gloves, pants))
*: Overcrafting is using a skill from one branch to allow you to craft over your level in another branch. recommended maximum overcraft is 20 levels. Digging over your level is not recommended at all. always dig no more than 10 levels above your skill, when you're given your next ability.
Magic Amplifier crafting is also popular, as there are many types of casters, and healing and elemental are critically important.
1-handed weapons are probably the less common weapons to craft. Again, it's easier to grind in a single 2H weapon skill and overcraft your 1H weapons.
There's also jewelry and shields.
Jewelry is like armor. Everyone needs it. With the tree splitting at 50, it means you have 200 levels to go per 6 item plans. 1,250 levels. Many people say it's annoying to level, as they use relatively few materials per craft. On the plus side, they only use two material types, so it's easy to keep track of what you need.
Shields... I don't know much about. They're only used for tanks that use 1H weapon. Like I said, 1H are less common.
