"Hide is broken!" "Everyone can hide, so what's the point of Assassins?!"
How often have you seen those two complaints? I'll start by addressing the first: "Hide is broken". Well, yes and no. There's some bugs with hide. Namely, sometimes after you kill a Mob, they continue to "Observe" you, preventing you from going back in stealth. Try to run around a corner, out of LOS (Line of Sight), and you should be fine. Also, if you are in a group and your group is in combat, you are also considered to be in combat and cannot stealth--even if you are miles away from them. The later might have been intentional, but is frustraiting.
Lastly there is a third bug which I've expereinced once, and sadly forget what seemed to cause it (It had to do with zoning) where I couldn't stealth at all for about 6 minutes. Others have reported more freqent experience with this, though I personally have only seen it the one time.
I know what you're thinking... "But my stealth breaks every time I try to attack a Mob!". In Age of Conan, it is extreemely difficult to stealth attack in broad daylight. Is that broken? No, I think that's what they intended. Certian types of Mobs seem better at breaking stealth--namely animals. Again, I think this was probably intended. Even if you're visually hidden, animals have increased sense of hearing and smell.
Disclaimer! Supposedly people have been reporting that stealth has been "slightly nerfed" in the 6/09/2008 patch! That was less than 24 hours ago, so I've had a total of about 30 min playtime since the patch. I didn't notice much difference, except perhaps an increased sucess rate when closing in from behind. It's hard to say if anything was really chanced, or if it's the "placebo" effect.
Anyway, now for the tips:
-In instances and anywhere that's "dark", use your light meter. It's the little diamond below your mini-map: the brighter it is, the more light you're in. It's not perfect, but it'll give you an idea. Daytime, you're almost always shown to be in full or near-full light.
-During the day: Try to avoid direct Line of Sight with any mobs. As of the last time I played, even "gray" non-hostile mobs would still spot you and bring you out of stealth (something I personally find highly annoying, but hard to classify as a bug, as it may have been intended). If you can use cover (houses, boulders, hills, whatever) to keep inbetween you and your target Mob, then do so.
-During the day: Don't be afraid to unstealth and use your crossbow to pull mobs one or two at a time if you have to. People cry about not getting the SA (Stealth attack) off, but honestly that's just one hit; you can do the same damage with your combos.
-Use stealth to scout the area: Even when you can't use stealth to get those opening attacks against Mobs, don't ignore it completely. Assuming you're on a PvP server (which you should be, dagnabbit!), you're always at risk from other players. Use stealth when moving around the map. When you get to your quest area, take a minute to see if anyone else is in the area aswell. Look for loot-bags or other signs that someone was recently there. Try to listen for other footsteps, if someone's in stealth. You'll never be 100% safe (isn't that the point of a barbaric, bloodthirsty world?), but it reduces the frustraition.
The rest is just "play and feel"...if you're patient and you keep at it, you will get a feel for when stealth is going to work, and when it won't. And you'll learn to use it as an advantage. Hey, if I can get it, anyway can.
...lastly I should address that second complaint that I mentioned in the opening. "Everyone gets stealth!" ... well, sort of. Keep in mind that non-rogue classes *can* hide, but move extreemely slow and it uses stamina to move in hide. A griefer can hide and stand still next to an NPC or whatever and wait for hapless victims to come along. Not much you can do about that. But it's pretty obvious that it's not very effective for them to stay stealthed all the time. Other classes don't get the Stealth Attack bonus (not to the extent that we do), and if they try to stealth up to you for PvP, they'd be left with depleted stamina. Why do you think that you see so many people running around unstealthed even though we all can hide?
Granted rangers and barbarians can hide the same as us. But, like it or not, they are rogues too.