Outlook, how do I hate thee? Let me count the ways.
ZDNet has a good rant and discussion of Outlook frustrations. Here are a few of mine:

• Can't drop a contact in the To field of a new email message
• Forgets any aliases I don't use after few weeks -- frustrating
• Rules management sucks...you end up with way too many rules
• Rules performance is horrible. They take forever to run and won't run in the background
• Outlook runs rules when mail "arrives"...but not if it "arrives" while Outlook isn't running.
• Takes forever to start up over dialup. Has to process all reminders, events, tasks, etc. before you can get to your inbox. (Hint to MS developers: multitasking means some tasks can run in the background.)
• HTML email authoring is a mystery...where do I enter my code, please?