Tasks is definitely not an Outlook or Remember the Milk replacement if you need any advanced features. One glaring omission is no synchronizing with a calendar on a Blackberry, in Outlook, or not even with other Google products like Gears and Google Calendar. Since I suspect that many potential Tasks users also use Calendar, not having a link between the two seems like a big barrier to wide-spread adoption. Speaking of the due date, nothing happens if you miss it. No alarm, tasks turning different colors, or messages of impending doom.
There are also no task priorities or filtering. Another biggie is the lack of reoccurring tasks. This is one of the biggest advantages of using a digital to-do manager over a paper one. With digital you shouldn't have to rewrite each week to water the plants.
Google has some wonderful collaboration tools like Documents, Notebook, and Google Calendar. Unfortunately, Tasks isn't one of them. You can't share a task list with a colleague or spouse, and you can't email someone a task like you can Outlook.
You can print out your list, but it's not intuitive. To print in Firefox with Tasks in it's own window, right-click and select Print Frame from the This Frame menu. The result isn't bad, but nothing special.
I also suggest using Tasks in its own window instead of in the Gmail screen. When I had it in Gmail, my browser slowed down considerably. Speed was normal when Tasks was in its own window. I also didn't like that when in Gmail, Tasks covers up most of the messages on the bottom right.