Ah, the good stuff. Let's not waste any time.
1. The UI is fast, really fast. My gamer's eye can occasionally pick up some stuttering, but in general the OS is very responsive.
2. The web browser is fast, really fast. While this is the first smartphone I've owned, I've used many others and have owned many phones. This one has some of the best page loading speeds I've seen.
3. The 3.7" screen is huge, crystal clear, the glass is a boon, the resolution is awesome, and the touchscreen aspect is almost too responsive. Text on it is razor sharp, and when viewing webpages, sometimes I don't even need to zoom in to read them.
4. Having a dedicated keyboard is nice. While the on-screen keyboard works really well, it's not so good in portrait mode and it's still better to have a physical backup. The placement isn't the best, and as stated before the control pad (aka "arrow keys") leaves much to be desired, but overall its a win.
5. The built-in wifi means I can piggy back onto my comcast connection when I'm at home or near a hotspot... and it makes the internet browser that much faster. I don't really use Youtube, but the videos pretty much load instantly.
6. There's a convenient widget that allows me to toggle the wifi, bluetooth, GPS, auto-sync, and brightness settings on the fly - two thumbs up.
7. Google Maps works great with the GPS, and being able to voice search a location directly within the app is badass. I searched for a McDonald's near where I was driving the other day... even though I already knew where it was. It's like the phone is my own personal attendant. Now if it could just do my laundry...
8. Speaking of GPS, having a built-in GPS vs an app+location is far more convenient. The gps feature on my Dare was dreadfullly slow.
9. The Google App Market... with 10,000 apps (and steadily growing), I've found plenty of handy/entertaining apps to download so far, and I've only had to pay for one of them. Sure, the iPhone has over 100k apps, but let me ask you this: Do you
really need an app for that, let alone over 1000 apps for that?

I would like to see better sorting/searching options in the market, however. Right now all you can do is list them by popularity, or search by name.
10. While I wasn't impressed with the camera itself, the video side of it was pretty good.
11. The Google Voice App. I have not yet installed this, but it was one of the features that pushed me into the Droid over some of the other smartphones in Verizon's lineup. It will allow me to integrate my Google Voice phone number into the phone, and use my unlimited web browsing to send unlimited free SMS and use visual voicemail without having to pay for those features. Currently I have a $5/month for 250 messages plan that I'll be able to drop, and Verizon's Visual Voicemail app requires a $3/month fee. That's $8/month I won't have to deal with, further reducing the gap between my new and previous bill. The only downside is that once I figure out which one of my two Google Voice numbers I'm going to use, I'll be superficially switching my number... and switching numbers is always a pain in the ass.
12. Gmail - The Droid has you sync a gmail account with your phone when you first fire it up. This allows you to use the gmail app, and load your google contacts directly into the phone. Regardless of whether someone has a phone number, they'll be in your contacts list so you'll still be able to email them directly from the phone. I admit it seemed odd to me at first, but now that I've used it and wrapped my head around it, it makes a lot of sense. You can change the contact list to display just contacts with phone numbers, but I left it alone since I don't do a lot of phone chatting to begin with (plus it makes it feel like I have more friends that way...

). Also, you can sync additional gmail accounts as needed... I have a secondary gmail account that I use for professional purposes - applying for jobs and the like. You can switch between them inside the gmail app, and when you get an email it notifies you from which account it lands in.
I'm sure I could find lots of other little things to list... but I think that's enough. I was going to post some pictures, but they came out shitty, and there's plenty of them floating around the web. I might throw up a couple later with some of the games/emulators.
Overall and despite some annoying quirks, I'm very happy with the phone.