Other than task and voltage level, are there further parameters which separates Hazard Risk 2 from 2*? For example incident energy levels, etc.?

Take a look at this existing thread. It may answer (or further confuse) your question: http://www.arcflashforum.com/showthread.php?t=384 .

TxEngr

the main difference is the PPE requirement Category 2* requires an arc-rated hood, where as Category 2 you can just use an arc-rated face shield.

It looks like 2* goes away with the 2012 Edition of 70E. A bit of confusion is finally resolved by that one.

(See 70E-A2011-ROP.pdf)

The 2* naming goes away, but the new requirements for HRC 2 will be those of the old 2* (arc-rated hood or arc-rated balaclava and faceshield now necessary in HRC 2, with a minimum arc rating of 8 cal/cm^2).

Now that's confusion in progress, with all the labels in the field which will still call for HRC 2, or worse, labels with the PPE listed for HRC 2 at the time the label was affixed to the equipment.

Behind the scenes it actually makes sense - it's just one more change to get used to and yes, short term confusion in the process. NFPA 70E requires parts of the body within the AF Boundary to be protected and there was a conflict in that Category 2 does not necessarily accomplish this since there could still be exposure to the back of the head. Category 2* did.

The "New 2" attempts to resolve this conflict.