Words unique to a user - where not in

I have a very simple table structure here. Just a list of words related to a simple user_id.

Word Table: 
    word - varchar(50)
    user_id - integer

I need to find words used by one user that are not used by other users. Currently I'm doing this and it works alright on Postgresql (9.0.3) at 200k words (~.3-.5 seconds) and completely falls over on MySQL (5.1.54) with the same data (5+ mins and it is still running). All used columns are indexed.

SELECT  
    word, count(word) as count
FROM 
    words
WHERE         
    word not in (select word from words where user_id <> 99 group by word)
    and user_id = 99
GROUP BY word
ORDER BY count desc LIMIT 20

1) Anyone know of a better way to do this?

2) Anyone know why it is completely failing on MySql?

EDIT: This fixes the issue on MySQL, from 5 mins+ to 10-20ms - Thanks Borealid

SELECT  
    word, count(word) as count
FROM 
    words
WHERE         
    word not in (select distinct word from words where user_id <> 99)
    and user_id = 99
GROUP BY word
ORDER BY count desc LIMIT 20

Thanks.

Try NOT EXISTS():

SELECT  
    w1.word, 
    COUNT(w1.word) as count
FROM 
    words w1
WHERE         
    NOT EXISTS (
        SELECT 1 
        FROM 
            words w2 
        WHERE 
            w2.user_id <> 99 
        AND 
            w1.word = w2.word 
        )
AND 
    w1.user_id = 99
GROUP BY 
    w1.word
ORDER BY 
    count DESC 
LIMIT 20;

Make sure you have an index on the user_id and word (or a combination), use explain to see the queryplan and what works best for you.

====== Edit: Also try the LEFT JOIN solution using a IS NULL:

SELECT
    w1.word,
    COUNT(w1.word) AS count
FROM
    words w1
        LEFT JOIN words w2 ON (w1.word = w2.word AND w1.user_id <> w2.user_id)
WHERE
    w1.user_id = 99
AND
    w2.word IS NULL
GROUP BY
    w1.word
ORDER BY 
    count DESC 
LIMIT 20;

Try an index on both columns:

CREATE INDEX idx_word_user ON words ( word, user_id);