As an expansion, I'd say it would be lacking (even compared to Warcraft).
There simply needs to be a lot more content (5mans, raids, chronicles, warfronts, dynamics).
After playing the levelgrind through multiple expansions in WoW, I can seriously say I'd be fine with leaving the level cap at 50 indefinitely.
The real question imo tho... is whether it feels like Rift has delivered an expansion's worth of content since launch. IMO, it has.
If they had 'held back' all the content patches, like WoW, and decided to deliver them all at once... (obviously it would have been bad for the game)... but what an impressive feature list it would make on the back of the box.