Also keep in mind that GameFreak don't make the games with competitive gameplay in mind. They're doing it for fun and profit (rather, the latter as caused by the former). They take some measures to prevent over-centralization or total brokenness, but in general, those are tertiary concerns.
However, as evident by Smogon's tiering system, they have done some blunders. I think Moody is the biggest one. It ended out more powerful than intended, I think. The Pokémon having it all end up invincible just by staying in.
Not far behind trails Stealth Rock. The idea was brilliant, but the execution... oh my. Stealth Rock ended up punishing only some Pokémon, shutting them out from even friendly multiplayer (the sort of metagame GameFreak cares the most about) usage, while totally ignoring others. It might be a balancing force in the metagame, we regard it as something that keeps Volcarona and Dragonite in check, but generally it screws over some Pokémon far too badly. Most notably the likes of Charizard or Yanmega. To put Stealth Rock bluntly, "it's not fair!".
Contrast Regigigas. It needed something to compensate for its mighty stats, so they added an ability that not only hindered it. However, somebody must have thought this wasn't enough, so they removed all sorts of recovery whatsoever, and Protect. Regigigas is not only hindered, it's neutered and urinated all over.
Anyway, drifting off-topic, I just wanted to say: Keep in mind that GameFreak aren't designing their games for us. They do it to satisfy their target group. They have to maintain in-game balance, they make the game so that every play-through doesn't end up the same. Kids battling their friends, or perhaps Global Link battles, are as far into competitive battling as they want to go to make some balance to it. In that regard, it's almost safe to say that we won't get another Stealth Rock. Imagine what an Electric version would do to Gyarados beside the original Rocks, for instance. Or how Volcarona would fare if another instant-damage entry hazard is added alongside the Rocks and Spikes. We'd be talking ZHKO. Fan-favourite Pokémon becoming practically useless, GF would hate to see that. Giving Stealth Rock so widespread usage was a mistake they won't do again. Don't ever expect it to return as a freely available move like a TM or a Tutor Move. At least not before they add more field-clearing moves (I think Gen. VI will bring some).
So yeah, expect Acid Spray and Quick Guard tutors. Of course we'll see a few more powerful moves too, like Sludge Wave or Hurricane or something. It's the way it has always been. I'm excited anyway.