SQL left join vs multiple tables on FROM line?

Most SQL dialects accept both the following queries:

SELECT a.foo, b.foo
FROM a, b
WHERE a.x = b.x

SELECT a.foo, b.foo
FROM a
LEFT JOIN b ON a.x = b.x

Now obviously when you need an outer join, the second syntax is required. But when doing an inner join why should I prefer the second syntax to the first (or vice versa)?