How so? They're regularly adding new content on a monthly basis, something other MMOs take 3-6 months (sometimes, almost a year) to deliver. Instead of finishing off the content and then waiting and waiting and waiting for the next push, there's always something new to do, or at least look forward to.
Maybe it's just me, but I'd rather have these regular, tasty morsels that keep me gaming rather than gluttonous content binges twice a year.
Anyways, the developers have said that an expansion isn't something they're actively working on at the moment, but I'm sure there's a team percolating ideas for one. There always is.