Agreed with Saytael. Meltas are the best Special Weapon in the game right now, pretty much across the board. Melta spam is what kills tanks for most armies, since the difference between AP1 and not-AP1 is absolutely massive. Tau are lucky in that our primary AT is still alive and well (the Railgun lost very little killing power in the shift from 4th to 5th, while most AT took about a 35% drop).
As to O'Shaserra? She was built to lead a gunline from a long way away from the enemy, where she can operate with relative immunity to damage and keep those Fire Warriors and Broadsides in line. She's still really, really damned good at that, it's just that Tau gunlines aren't going to do too well in a competitive environment anyways, which makes her much less useful. She does have one of the coolest models in our range, though, which is one of the reasons that DoW stole it for their Tau commander. IMO, we don't see Shadowsun anymore for the same reason we don't see Etherials much; they're really only good in Gunlines, and Gunlines aren't a good way to play Tau except at low points levels (where Shadowsun doesn't fit due to cost).