I've got a Hardrock as well, '09 or '10 can't remember right now. From what research I've done on upgrading the bike it seems be kind of a mix.
There are a lot of people that just love the frame and will dump tons of money into it but there are a whole bunch more it seems that say the frame isn't worth it and it's better just to upgrade.
Probably the best place to start is with a quality saddle. Then the front shock, but you will be into a front shock as much as you paid for the bike to get a good quality unit. A disc brake upgrade will likely cost a couple hundred for good brakes but do you really need them? You will need to wheels to accomodate the disc brakes and that's if your frame came equiped in the rear to handle disc brakes.
Most people that ugrade their hardrocks end up replacing everything but the frame. But if you build the bike slow and just upgrade parts as they break or wear out it should serve you well.
Personally I love mine. Maybe it's because it was my first "quality" bike, I'll likely end up just upgrading as things wear out.
Of course the upgrades are almost endless, seat stem, handle bar, wheels and everything else.
'09 JKUR. Metal Cloak overlines, TF 3" LA, 35" Nitto Trail Grapplers, Home built winch bumper, custom built sliders, RK Drag Link and HD Pro Tie rod, Adams front shaft, TF Big Brakes, Speed Bumps, Warn M8000