Simple Queries would gain a lot of value if you could have more freedom to define join (on) conditions. When you try to change the join condition (for example, adding another), it is automaticallty converted into a "where".
This is specially bad when you want to define outer joins because the behaviour of having a "where" condition and an "on" condition is different, as it may cause the join to behave like an inner join, even if you check for nulls.
A lot of times I had to convert very simple queries (for example, 1 left join with 2 conditions) to advanced because of this.
This would also be very useful in terms of SQL performance.