Well ...
I'm not a powers guru, but there are some cases where "mirrored" powers behave differently -- ferinstance, one or two powers have front-loaded damage Imperial side, while the Republic gets back-loaded damage. Same damage, same effects in both cases, but it's far easier to interrupt the damage component of the Republic power. A handful of little things like this initially gave the Imperials a tiny advantage in PvP.
Unfortunately, that tiny advantage meant a few more wins for the Imperials (53 : 47 on some maps, per the devs) which has meant that an increasing number of "Play To Win" PvPers switched to Imperial.
What we have now is a t***** problem to sort out.
Imperial is more attractive to play (literally and figuratively, Imperial animations are often splashier). This has led to a population imbalance. Which has led to Imperial farming Illum turrets on several servers. Which is going to throw PvP even more off-kilter as Imperials get better gear.
That's one of the odd things about MMOs -- players can tease out the most minute advantages from a PvP system and leverage those advantages to a surprising degree.
For those who don't see the big picture: having OMG awesome gear is great -- have fun using it in PvE 'cause you're unlikely to be seeing a lot of hot Imp / Republic action until BioWare starts fixing its mess.
Edit: Really? A rose has many thorns and is therefore "t*****" ... that gets bleeped?