People tend to hate changes in software, having invested a chunk of time in learning the previous version. That's the only reason I can see for, e.g., preferring Windows XP over Windows 7. However, there are some specific feature losses in the new Google Maps, including

1. No support for My Maps
2. No separate Navigate button
3. No Zoom buttons
4. No Latitude support (whatever that is)

I saw some real panic over item 1 online right after the upgrade dropped, as people who had set up events based on shared maps didn't know how to address the problem. (There are some options but they're workarounds.)

I don't miss the separate Navigate button, which always seemed weird to me: isn't navigating one of the things you do with a map? I do use two Navigate shortcuts that the old version allowed me to save to a desktop folder but they're not very important to me.

The loss of the Zoom buttons is trivial once you discover the finger gesture that replaces them. The problem there is that it isn't discoverable; you have to find it by accident or by reading the instructions. I'm a software developer and I learned long ago that few users, myself included, actually read the instructions.

I might miss Latitude if I knew what it was.

They've improved some of the map's features; for example, they've replaced the clumsy old menu-driven alternate route selector with a metaphor similar to that on the Google Maps web site, with alternates shown in grey and selectable with a tap.

Overall, I advise everyone to try living with the new Maps for a while before giving up on it. After two weeks, try to remember what you liked about the old version. My guess is that 95% of users won't be able to and will find they're perfectly happy with the new version.