The bad part is that there's no visual indication in the calendar application as to which calendar events are accepted or tentative. In Outlook, tentative calendar events are visually marked. In the Droid calendar, you can't tell unless you hover your finger over it. I think there needs to be some method of differentiation, or at least let the user set colors or other visual cues, to accomplish this. You can send calendar invitations to users from the calendar application.
Other apps like Facebook and the Gallery work well enough. Perhaps the weakest part of the Internet feel is the browser, which doesn't support media types like Adobe Flash. You may be thinking that Flash support is no big loss, but how many websites that you use everyday are starting to use Flash for menus and other interactive components? Flash is pretty important. It's available on the HTC Hero, but not on the Droid.
There isn't much security built in except to protect the applications and the phone itself. Any additional security will have to be added through software, though as already mentioned, anyone with access to the phone will be able to easily uninstall security related applications. This is another case where having a way to install an application would be helpful.
There is an authentication mechanism which is used to grant access to the phone. Android has a 3x3 grid that which you draw a pattern on. You can set-up the panel to show the pattern while you authenticate or not, the latter is an attempt to hide your pattern from shoulder surfing. It takes some getting used to, but once you get the hang of the drawing, it's pretty simple. Except for the smudges.
On a clean screen, one calculation results in 389,112 possible patterns. But like passwords, I bet the pool of patterns used in the field is far lower, largely due to lack of user imagination. For instance, one person pointed out how common the Zorro pattern is.