Actions that require large numbers of host (more than 8 in my experience) timeout now because of PHP script memory exhaustion. I've basically had to crank the PHP memory for scripts setting up to 500mb in order to update more than 10 hosts at a time. I never had to do this with 1.8.

For example, if I try and delete 10 hosts at a time via the server gui I'll be returned to a blank page with no output. The machines won't have been deleted.

In the Apache2 error logs I'll see:
[Thu Sep 13 22:55:06 2012] [error] [client 70.99.250.82] PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 76 bytes) in /var/www/zabbix/include/classes/debug/CProfiler.php on line 232, referer: http://bigzabbix.seomoz.org/hosts.php?....

I'm not sure what other information I can provide.

The server has 18 gigs of memory total and 7 gigs free at the time of running. In the PHP./etc/php5/apache2/php.ini file I have:
; Maximum amount of memory a script may consume (128MB)
; http://php.net/memory-limit
memory_limit = 512M

That seems to me an absurd number. Our current host count in Zabbix is about 700.