To my mind LO Starmie is one of the most dangerous pokemon in all OU. First of all, its insanely dangerous offensively. Against offensive teams, there is usually nothing which it can't 2HKO or OHKO; and it will outspeed nearly everything bar Scarfers. Its only bane is Pursuit and U-turn and it can close to OHKO all of the common users of those moves. Plus, just look at a list of OU's top 10 and you'll see that Starmie can beat virtually all of them
Stall and defensive teams don't usually have the same problem with being swept, courtesy of Blissey, as well as unusual stuff like Snorlax, Abomasnow, Lanturn etc. However, they also have no real answer to Starmie because it will just spin their hazards. Scarf Rotom and Sucker Punch Spiritomb are really the only spinblockers which can actually force Starmie out if they switch into a Hydro Pump.
To my mind, there are only a couple of actual things which can properly beat Starmie. CB Snorlax, Scarf Scizor (and maybe Band) although it still takes a crapload; and Kingdra, since it is just about the only pokemon which can actually set up stat boosts against Starmie with a ChestoRest set.Everything else is either exposed to massive risk, or still allows Starmie to safely spin away any hazards.
In addition, I don't even think people have realised the sweeping potential of Starmie yet. Having Toxic Spikes down, for instance, takes care of 90% of Starmie's bulky checks. That just leaves Scarfers which can easily be taken advantage of.