You have seen my top 5 and the discussion on those from Apple's website that have been posted in the last week or so. But I am sure you may ask, with all these great things, what may I not like in Leopard. Here are a few things to consider.
Classic - With Leopard, even if you have a PowerPC based Mac, Classic OS9 applications cease to exist. That's right, OS9 is finally gone. If you still have apps that only run in OS9, you better be thinking about your future plans.
Leopard requires more RAM - Apple says Leopard needs at least 512Mb but if the usually difference between required ram and what you really need exists, plan on 1GB to really make it usable.
Time Machine needs a separate disk - Yes you could partition your drive into two parts but what sense does it make to use a backup system that backups your valuable data to the same drive. That defetes the purpose. Purchase a $100 external drive and call it good.
Quick View is still a View - Sure, with quick view you can view word documents, excel spreadsheets, pages documents and many others but you will still not be able to edit them. Don't expect more than really is there in this cool new feature.
Is my machine big enough - Leopard calls for an Intel Mac, Power PC G5 or a Power PC G4 with an 867MHz processor or higher but be careful here. If your machine is on the fringe, perhaps you should be upgrading to Leopard in a new machine..... and if you have a G3..... your are well over what Apple considers overdue.