Not precisely sure where we were in 2011, but I think these are the biggest ones that came after, off the top of my head (i.e sure to be missing something):

  - Merging content and index clusters to one to make index clusters elastic and auto-recovering on data loss.
  - Fully realtime writes.
  - Support more advanced machine-learned ranking through tensors.
  - Streaming (personal) search supporting a large write rate.
  - Document references.
  - WAND and RANK operators.
  - Rank features over multivalue text fields.
  - Predicate fields.
  - Lots and lots of performance work.