some of my favourite riffs are v easy to play, south of heaven, seek & destroy are real easy to play but very catchy an great riffs. i suppose it goes down to whatever you consider to be a good riff though
A riff doesn't have to be complex to play to be good (most are though I would imagine).
From my listening perspective, a riff has to have a good sound to go with it. Speed helps in some cases too. You can't beat a nice heavy riff.
For a riff to be considered great it has to be original, it has to be memorable and be full of feeling. In my opinion a great riff does not have to be overly technical or demand a huge amount of skill to play.
Some of the best riffs ever are easy to play.