Here is a beginner guitar riff in Drop D ( tuned down a half step also)that is based on a repeating triplet sequence. Licks like this become more interesting on guitar when you start to get a bit more advanced rhythmically by incorporating 16th notes and rests. Some great examples of a very rhythmically interesting riffs can be found in the guitar work of Pantera by the late Dimebag Darrell. Pantera was a very rhythmically tight band. The bass, drums and guitar all embraced some complex rhythms and time changes to make their music as powerful and compelling as their songs lyrics. For more drop D riffs check out Black Label Society headed by the great Zakk Wylde. To my ears Black Label is like a modern day Black Sabbath on many of their slower riff based songs. Some great metal grooves here and some outstanding music by the whole band.
So try to get your fingers and head around this simple riff and then check out some more advanced material. One thing I find helpful is to hit a drum machine or drum backing track and try to come up with grooves that match the track. Drop D tuning is great for this because is makes it easier to turn the guitar into a simpler rhythmic instrument because of the layout the chord shapes take in the new tuning. You can play a lot with just one finger so this frees up your mind to concentrate on rhythm.
Another interesting exercise is to grab a drum rhythm book and assign each note value a note of chord. Start with maybe one or two notes and play the rhythms as written on the staff. This even works well if your not great at standard notation because you only need learn the rhythmic values of the notes.
