I upgraded from Zabbix 1.8.6 to 2.0.2 and did not experience any issues during the upgrade process.
I used Kodai Terashima's RPM packages from here: http://www.kodai74.net/packages/zabbix/zabbix-2.0/rhel/5/x86_64/

I noticed the new default templates were not present, so after searching the forums I found them here:
https://zabbix.org/wiki/Zabbix_Templates/Official_Templates#Zabbix_2.0.0

I was able to import all the App and SNMP templates successfully, however importing the OS templates fail (I have tried Linux, Windows, Mac OS X, and OpenBSD).

Here is the error I get when importing "Template OS Windows":

Created: Application "Zabbix agent" on "Template OS Windows".
Created: Item "Host name of zabbix_agentd running" on "Template OS Windows".
Created: Item "Agent ping" on "Template OS Windows".
Created: Item "Version of zabbix_agent(d) running" on "Template OS Windows".
Created: Trigger "Host name of zabbix_agentd was changed on

is unreachable for 5 minutes" on "Template OS Windows".
Created: Trigger "Version of zabbix_agent(d) was changed on

" on "Template OS Windows".
Created: Application "CPU" on "Template OS Windows".
Created: Application "Filesystems" on "Template OS Windows".
Created: Application "General" on "Template OS Windows".
Created: Application "Memory" on "Template OS Windows".
Created: Application "Network interfaces" on "Template OS Windows".
Created: Application "OS" on "Template OS Windows".
Created: Application "Performance" on "Template OS Windows".
Created: Application "Processes" on "Template OS Windows".
Created: Item "Average disk read queue length" on "Template OS Windows".
Created: Item "Average disk write queue length" on "Template OS Windows".
Created: Item "File read bytes per second" on "Template OS Windows".
Created: Item "File write bytes per second" on "Template OS Windows".
Created: Item "Free memory" on "Template OS Windows".
Created: Item "Free swap space" on "Template OS Windows".
Created: Item "Number of processes" on "Template OS Windows".
Created: Item "Number of threads" on "Template OS Windows".
Created: Item "Processor load (1 min average)" on "Template OS Windows".
Created: Item "Processor load (5 min average)" on "Template OS Windows".
Created: Item "Processor load (15 min average)" on "Template OS Windows".
Created: Item "System information" on "Template OS Windows".
Created: Item "System uptime" on "Template OS Windows".
Created: Item "Total memory" on "Template OS Windows".
Created: Item "Total swap space" on "Template OS Windows".
Created: Discovery rule "Mounted filesystem discovery" on "Template OS Windows".
Created: Discovery rule "Network interface discovery" on "Template OS Windows".
Created: Item prototype "Free disk space on $1" on "Template OS Windows".
Created: Item prototype "Free disk space on $1 (percentage)" on "Template OS Windows".
Created: Item prototype "Free inodes on $1 (percentage)" on "Template OS Windows".
Created: Item prototype "Total disk space on $1" on "Template OS Windows".
Created: Item prototype "Used disk space on $1" on "Template OS Windows".
Created: Item prototype "Incoming network traffic on $1" on "Template OS Windows".
Created: Item prototype "Outgoing network traffic on $1" on "Template OS Windows".
Undefined index: expression [api/classes/CTriggerPrototype.php:1139]
Undefined index: expression [api/classes/CTriggerPrototype.php:1141]
Cannot implode expression "".

The last three error lines are the same when importing other OS templates. I also get the same error when importing a user-created NetApp template from here: http://www.zabbix.com/forum/showthread.php?t=26466

I don't see any obvious errors in zabbix_server.log or Apache error_log