For those that may only vote and otherwise lurk, there's a decent amount.
The inability to create multi-communities/reddits (or feeds as Piefed calls them), the absence of post-folding/deduplication for when someone posts the same article to multiple communities (sometimes similar, sometimes distinct), the absence of keyword filtering to automatically filter out stuff from local/all feeds one's uninterested in, and these are just a few from the top of my head for those that mostly lurk.