Spinks correctly points out that stealth is tricky to balance, and not so great in group in WoW. Unless of course you're trying to sap one, ambush the other, and blind the third, which is merciful because then they only hear the screams while the sapped one has to watch.
What's so bad about stealth in WoW? With the amount of AoE, it's trivial to get knocked out. It can be tricky to get back into stealth in PvP, but short of vanish, it's effectively impossible in PvE.
I propose two simple changes to improve stealth in PvE.
1) If a rogue or druid is not the current target of any mob and has avoided combat for however long (whatever the criteria would be in PvP to drop combat), they can stealth. If they've generated any aggro they can still be attacked, but only after non-stealthed players. This means that PvE combat doesn't start with sneaky backstabbing and end with standing right out in the open stabbing away with absolutely no sneakiness. Clearly assassination rogues would need retuning of their energy generation, since the regen increase would be up much more often.
2) Stealthed players will not be attacked by mobs unless they are specifically stealth-detecting or there are no non-stealthed players around. This is brought on by halls of reflection, in which I like to stay a bit away from the group so I can take out a mage, but then a footman inevitably attacks me.
Rogues need more reasons to be in stealth, reasons beyond just damage. Imagine if cheap shot was a stun and a 50% damage output reduction for two seconds, similar to how hammer of justice has an interrupt component. This would be a huge help on many bosses and shift rogues a bit away from being a pure damage class, standing around looking like an enhancement shaman with fewer spells.