Support multi effect, 3 built-in themes: default, transparent and box. Easily create and customize themes
A problem I noticed was that slideshow only plays nice if loaded into a module position allowing full width of the template... Putting another module to the right of this one will not work... at least not in Gantry because it forces anything next to it to go under it.
Also there is no way to turn off the slides from linking to the k2 item pages... so forget about making the slideshow independent of the rest of your site without hacking the code.
Now the coolest thing about it is that it re-sizes images to the width specified. Just make sure your k2 setting of XL is set to as wide as the slideshow. It then crops the image top and bottom to fit your size... awesome.
Ability to turn off linking/clickable - so it's a static slideshow.
Ability to set the link to a URL of choice, based on an extrafield, rather than linking to the actual item. If that extra field is unused it is not clickable. Then you would have a slideshow that's clickable, but items link to pages other than the slide items.
Ability to load into module positions next to other modules without it breaking the template layout and forcing the modules to its right to be below it.
Do these things and I would easily pay $50 for it... or $100 for an annual dev license, because it would be the absolute best slideshow tool for k2 or Joomla out there.