Tested on Aqua Data Studio 14.0-dev-47 Build #: 32127 on Ubuntu 12.04( Mongo DB 2.4.3)  and Windows XP ( Mongo DB 2.0.9).

1. Alias for FROM( subselect) without AS word gives error. See first attached screenshot

2. In documentation FROM in the SELECT statement shown in brackets [] as optional. But SELECT 1 does not work

3. Using  operations ( +, -...) with <ALIAS>.<field name>  comming from result of  FROM (subselect) as ALIAS gives error. See second attached screenshot. <ALIAS>.<field name> without operations works correct.

4. Strange that statement: SELECT _id  FROM( SELECT city FROM baseball) return next unique _id from sequence instead null as _id is not present in subselect result set. See third attached screenshot. Also _id returned even if in subselect used not existed column.  E.g.  SELECT _id  FROM( SELECT  notexisted  FROM baseball)

5. If in FROM subselect used one more level of subselect - empty result set returned instead rows. E.g. SELECT valuation FROM (SELECT valuation FROM (SELECT valuation FROM baseball)) If it is not possible then necessary note this in docs. Interesting that using many levels of nested subqueries in WHERE clause works. E.g. SELECT valuation FROM baseball WHERE valuation IN(SELECT valuation FROM baseball WHERE valuation IN(SELECT valuation FROM baseball ))