Is it worse than other flus, or just more unexpected?
H1N1 is a new strain in humans and so it has the potential to be extremely virulent, which is common when a virus jumps between species.
A well-adapted virus will keep its host healthy enough to continue being a nice place for the virus to live so it can replicate and go on to infect others. So influenza (which is native in birds) won't make a bird very sick, but when a strain mutates and is suddenly able to infect humans, it has the potential to be extremely deadly because it's not really adapted to our species yet. (The common cold? Really really well adapted to humans. Ebola? Not.)
Influenza is worrying because (a) it mutates a LOT and (b) is extremely contagious and (c) doesn't really have a cure (most flu treatments are geared towards preventing a secondary bacterial pneumonial infection).
Fortunately, it's looking like H1N1 is not very virulent and will probably go away on its own.
On a side note, did you know that in the 1918 flu pandemic, the majority of fatalities were NOT from secondary bacterial pneumonia, but from an unusually violent immune reaction? (Hence the larger numbers of young people dying than in a "normal" pandemic - the people with the strongest immune systems had such a strong immune response to the virus that they basically exploded their own lungs with white blood cells. Fun fact!)
t /flu geek
[eta: And holy crap, I type slow. Teppy got there before me AND used big words like cytokine storm!]