Stealth is a very controverial ability, very strong while used and very weak since we cannot use often.
@Sanguinezor: the "pro" reasons you bring are very very feeble. The dps increase provided by shadow focus is neglegible - it's just that there is no better option available. Also, using Vanish as a dps cd (like it was with Overkill) was something Blizzard wanted to avoid, but they didn't get the job correctly since the use of vanish in PvE is tied to a free opener/CP or FW applicationfor Sub.
Obviously Stealth is a clear PvP tool and the advantage to run around unseen is great to setup a strategy. Still, our openers are for control (Garrote and/or CS are way superior than an Ambush) and after that, stealth isn't that useful - Vanish is good for resetting a fight, not for the stealth itself.
A strionger stealth in PvP imho could lead only to imbalances; anyway a bigger use of i in PvE would be cool since there's much more spacing in fight mechanincs without screwing other classes. If we cannot use out control tools and the dps dosen't matter, give us some special tasks to do
The biggest error is that it is not sneaking around but outright invisiblity, but apart from that it is a ludicrous tool so you can get into position and initiate the attack.
In PvE the tank ensures that you can get into position without getting attention to yourself so it is not really needed. Some fights could be designed that for some parts you could use sneaking as plan B, but that is not the "brokedness" of stealth but the encounter design.
In PvP you have openers that influence the outcome of the battle, when initiation of the fight is a great advantage already if you can use it to your advantage. The ability to reset the fight and reinitiate on a whim is no offense but would be broken the same way as stunlock was as you could avoid retilation from your opponent. Oh look the silence from garrote just wore off, time to put it back, that caster may try something funny...
About being able to do shaenigans while stealthed, I can't see where it would not conflict with the current game mechanics in PvE. In PvP the people would notice that suddenly they are poisoned kind of betraying yourself, not to mention that would make you able to weaken them severely without putting yourself into too much risk.
In a nutshell Stealth is a tool to initiate a fight and not to solve all your problems.