[Andreas]

OK, so let's take a further look at your queries

SELECT id_empfaenger
FROM sms
WHERE id_empfaenger  = 2

Dunno what this is, probably some instant messaging hack?
IMHO useless.

SELECT user.avatarid, user.avatarrevision, avatarpath, NOT ISNULL(avatardata) AS hascustom, customavatar.dateline
FROM user AS user
LEFT JOIN avatar AS avatar ON avatar.avatarid = user.avatarid
LEFT JOIN customavatar AS customavatar ON customavatar.userid = user.userid
WHERE user.userid = 2

Used to fetch your avatar (and display it).
It's up to you to decide if this is necessary

select count(*) as new_posts from post where dateline > '1106285277'
select count(*) as new_topics from thread where lastpost > '1106285277'

New posts and Threads since last visit..

select count(*) as all_users from user

Total number of users.
Now tihis is a total waste, as the number of users is in the datastore and could be aquired without a query.

select count(*) as all_posts from post

Total number of posts. Alos a waste as this figure is available in index.php as $totalposts

select count(*) as all_topics from thread

Same here for threads => $totalthreads

SELECT * FROM quotes WHERE approved = 1 ORDER BY rand(877250764)

Guess that's your random quote hack ...

SELECT template
FROM template
WHERE templateid = 5017

Uncached template, already solved.

SELECT * FROM `irc_channel`

Some "users in chat" hack?

SELECT userid, username, posts FROM user ORDER BY posts DESC LIMIT 0, 5

Top 5 posters display?

SELECT userid, username, posts, joindate FROM user ORDER BY joindate DESC LIMIT 0, 5

The last 5 newest members.
If you want this, I think it would be better to put this info in datastore when a new member registers.
As the last newest member is already being stored I think this could be done without additional queries, it just needs code adjustments.

SELECT COUNT(*) ,
SUM(views) AS hits
FROM  thread

Totel thread views

=> There are a few things that can be optimized without loosing features.