They're cooler! 

Lol, nah, the HDS will support a whole slew of add-ons from Sirrious to Sonic Hub.  The broad band sonar is a big improvement over the LCX if you ask me.  There's others, but these are the ones I'm familiar with off the top of my head.  Then there's screen resolution, the hot keys, bigger hard drives, insight, etc... And I'm sure there's others.  But don't discount the LCX.  I used LCX for a long time and actually hated to see them go, as I felt they just had the bugs worked out!  The problem with LCX is that you're going to have a hard time getting them serviced by Lowrance now.  I had a 27C that just last summer crapped itself and when I called, it was going to be $320 to fix it...or they could sell me an HDS7 for $500.  Their support for LCX is going to be drying up very quickly.