One nifty feature of Postgresql is that if you have a group by a,b,c,d, you need to aggregate everything else in your select that isn't a/b/c/d, or the statement won't prepare, whereas if you don't select an aggregate in MySQL you'll just get one random column, which is probably not what you want.
This is mostly true, but in recent Postgres versions (since 9.1 I believe), if you GROUP BY the primary key, you can still include other columns in the SELECT statement. For instance:
SELECT a.name, COUNT(b.id)
FROM a
LEFT OUTER JOIN b ON b.a_id = a.id
GROUP BY a.id
