criteria uses "inner join" instead "left join" approach by default making my query work not the way I planned

Tested and working solution:

    def result = A.withCriteria{
        createAlias('someObject', 'so', CriteriaSpecification.LEFT_JOIN)
        createAlias('so.importantObject', 'imp', CriteriaSpecification.LEFT_JOIN)
        or {
            isNull('so.importantObject')
            eq('imp.interestingFlag', false)
        } 

    }

Solution update as suggested in comment:

    def result = A.withCriteria{
        createAlias('someObject', 'so', JoinType.LEFT_OUTER_JOIN)
        createAlias('so.importantObject', 'imp', JoinType.LEFT_OUTER_JOIN)
        or {
            isNull('so.importantObject')
            eq('imp.interestingFlag', false)
        } 

    }

Use left join to achieve this. This should work, but I haven't tested it live.

def result = A.withCriteria{
    someObject {
        createAlias("importantObject", "io", CriteriaSpecification.LEFT_JOIN)
        or{
            isNull('importantObject') // conditional 1
            eq('io.interestingFlag', false) // conditional 2            
        }  
    } 
}

Eidt: based on this post: http://grails.1312388.n4.nabble.com/CriteriaBuilder-DSL-Enhancements-td4644831.html this should also work:

def result = A.withCriteria{
    someObject {
        isNull('importantObject') // conditional 1
        importantObject(JoinType.LEFT) {
            eq('interestingFlag', false) // conditional 2
        }  
    } 
}

Please post your results on both solutions.

Edit 2: This is a query that is exactly a situation you've described, but differs from your examples. You must start from here, use showSql to debug generated SQLs and fiddle with left joins.

def result = A.withCriteria{
    or {
        isNull('someObject')
        eq('someObject.importantObject.interestingFlag', false)
    } 
}