First of all, let's talk about the plot, or lack thereof, in the Riddick movies. It's like they took a bunch of random sci-fi clichés, tossed them in a blender, and hit the "disaster" button.
That's a common complaint among people who haven't actually watched them. Look at Chronicles of Riddick for example... it's LITERALLY Macbeth with Riddick in the role of Duncan. Of course to get that, you'd have to be familiar with Macbeth.
And let's not forget about the acting, or should I say lack of acting. Vin Diesel's impressive range of facial expressions consists of a permanent scowl and a look of constipation.
The first time I ever saw Diesel was in his short film "Multi-Facial", his performance there is DEVASTATING. Clearly he CAN do more than what he's given. Iron Giant was phenomenal. I've heard nothing but good things about Boiler Room but I haven't seen it.
Multi-Facial can be seen on Youtube, worth a watch: