After checking out the screenshots of Microsoft's Dock implementation in Windows 7 I think it's a step in the right direction for them. It's a little embarrassing to have to admit your main competitor was right a decade ago (2 decades if you count NeXT) but Microsoft has always done best when they copy something that works and add a few useful features on top of it. That's exactly what they're doing here and it's a pretty good implementation of the Dock. In fact, it's probably better than the OSX Dock which hasn't seen any significant upgrade in a while. The changes in 10.5 were largely irrelevant to how the Dock actually works.
What's wrong with the taskbar? Everything? It was fine back in 1996 when you had maybe 6 or 7 applications open if you were a power user and could afford 64MB of RAM to handle it. Today it's not uncommon to have dozens of windows open that you must actively switch between to complete a task. The taskbar simply doesn't scale and is not consistent. I could go into that more but it's pretty obvious so figure it out.
Managing this type of mess on a taskbar is nearly impossible. I've tried. It drives me insane. The lack of Expose doesn't help but that's another issue entirely. (the screenshot cuts off my second 20" display which is also full of windows) The Dock works by saving space combining launching & running tasks and always combining all associated windows of an application together. The bouncing Dock notification is also very useful as extra information can be included in the Dock page (such as transfer speed, unread message count, etc)
Having not used the Windows 7 Dock (only looked at it) I can't really comment on it outside of saying it looks like an improvement on the OSX Dock. The simplicity of the OSX Dock is nice but when you're dealing with say 8 Terminal windows a static text list of their names is less useful than a graphical thumbnail. The JumpList feature is somewhat available by right clicking a Dock icon but not implemented nearly as well and with flexibility.
Anyway, I'd like to see Apple do two specific things to improve the Dock:
1) Allow widgets to run in the Dock in the same style as avant-window-navigator on Linux. Avant started as a straight OSX Dock clone but has got so many new features via widgets that it surpasses the original. Different Dock icons could work as containers. So for example, all my audio apps could go into one container which would spiral out in the style of Stacks to show what was inside. A Twitter widget could popup a simple text entry bubble with a Submit button. Etc.
2) Thumbnail previews would help. Hover for a few seconds and get an Expose style pop-up of each window.
That's about it. Otherwise I think the OSX Dock is very functional and to the point. It would be a shame to see Apple let Microsoft beat them at their own game. The Dock is long over-due for some tweaks. The changes in 10.5 show all the fundamentals are there to make these changes. Someone just has to pull the trigger and decide the 10 year old Dock needs to be revised. We certainly have the GPU power for it. These options could all be off by default since the basic Dock configuration is fine for most people.