Ordered Aggregates. This is extremely useful for spatial aggregates and ARRAY_AGG, STRING_AGG, and medians where you care about the order of the aggregation. Will have to give it a try.
For example if you are building a linestring using ST_MakeLine, a hack you normally do would be to order your dataset a certain way and then run ST_MakeLine. This will allow you to do
ST_MakeLine(pt_geom ORDER BY track_time) or ARRAY_AGG(student ORDER BY score) This is very very cool. Depesz has some examples of ordered aggregates.
Join removal -- this is a feature that will remove joins from the execution plans where they are not needed. For example where you have a left join that doesn't appear in a where or as a column in select. This is important
for people like us that rely on views to allow less skilled users to be able to write meaningful queries without knowing too much about joins or creating ad-hoc query tools that allow users to pick from multiple tables. Check out Robert Haas why join removal is cool for more use cases.
GRANT/REVOKE ON ALL object IN SCHEMA and ALTER DEFAULT PRIVILEGES. This is just a much simpler user-friendly way of applying permissions. I can't tell you how many times we get beat up by MySQL users who find the PostgreSQL security management tricky and tedious to get right.
Of course you can count on Depesz to have an example of this too Waiting for 9.0 - GRANT ALL