Riff 2 sees our first change of meter from 4/4 to 3/4 and then back to 4/4. Notice how the change of meter is very subtle, that is, it doesn't scream out at you.
The 16 note run in bar 15 should be played very tightly and cleanly. Listening to the drums here should help you.
Rhythmically, bars 18-23 will feel a little odd at first but with practice, will become instinctive. I suggest that when practicing this part, you omit the bottom string and count the upper chord only. This will really help you to ascertain where the beats lie. Once you have that under your belt, you can add in the lower string and voila! There you have it.
Once you have that rhythm part down, keep it close to hand as it rears it's head a little later in the piece but in a different harmonic guise. Lets head on to riff 3!