As for the pros and cons of the D4...
One thing that's great about my D4 is that my family and friends can hear my voice better when I talk to them. but I think my D1 just had some hardware/microphone issue that I wasn't aware of in time to exchange it or replace it with my warranty. (i.e. I don't think my microphone issues were the norm, with my D1)
The external speaker of the D4 isn't quite as loud as it was on the D1. The D1's speaker was AWESOME. But if I recall, it seemed louder than the Rezound, when I tried that phone.
The vibrate is likewise quieter and less noticable. i almost never feel or hear it in vibrate mode if i get a call while the phone is in my pocket. with my D1, it didn't matter if i was walking, because i mostly could hear and feel the vibrate ring anyways. But still, it's not as abysmal as the Rezound's vibrate was, when i tried it for a weekend.
The Keyboard... I'm having trouble adjusting to the new physical keyboard. All of the keys were set off to the left because of the D-Pad on the right of the D1, but on the D4, they're more centered. It just takes a little getting used to. I thought I would hate the arrow buttons instead of the D-pad, but i've had absolutely no trouble adjusting to that. And I miss having the backspace button be in the very top right corner. But, I love having the 5 rows of keys (dedicated number buttons). Here's an interesting thing: if you click two adjacent keyboard keys at nearly the same time, it only acknowledges the first button you push. i guess it's to avoid typing errors. it's weird, though, because the audible feedback tells me i pushed two buttons, and so i think i need to hit backspace... but i don't. that'll take a little getting used to, too. lets see... what else... Oh, I don't like the caps lock button. I can still double-click the shift key to get into caps-lock mode. So it feels redundant, and I wish the Shift was where the caps lock is (because that's where I'm used to it being.
(as for an upside on the D4: it has Swype. now that i've given that a try, I really like it and can see the point of people saying that physical keyboards aren't so necessary. I still love using the real keyboard for landscape typing, though.)
In a dark room, the screen is still quite bright, which is a little hard on my eyes if I'm looking at my phone for a long time.
I haven't had any troubles with the battery dying early. it seems to last just as long as my D1 battery did... I'm pretty satisfied with it. (and I run on 4G all the time) but then again, I don't use my phone heavily throughout the day.
I never really gave much thought to bloatware, previously, but i find that it is annoying. simply because the various apps want to get updates from the market. but maybe i'll root soon, and freeze it.
Coming from CM7 on my D1, motoblur is a bit of a disappointment. i miss the tweakability of CM7.
This one's silly, but... when i'm browsing the web, i like to open the keyboard and hold the phone in my right hand. I used to be able to tap the menu button and then the Windows button with the thumb of my right hand on the D1. But on the D4, it's kinda awkward to stretch my thumb that far (because of the larger screen size). But oh well. i can still manage it, if i need to, in a pinch. and obviously everyone's thumbs are different lengths... so you may not have that issue. But in general, i quite liked the size of my D1.
Lastly, I HATE the screen display. i see way too many pixles (or sub pixels?), and i wish i could have the screen of my D1 back. I don't even care if the resolution of the D1 wasn't as high. (what happened to having just a pure, solid green? like in the battery icon? what's with the criss-crossing affect of the colors? yuck. it looks like a color photo in a cheap magazine/newspaper.) Because of the screen ugliness, now that i use swype, I'm almost tempted to trade my D4 in for a Rezound... (if that had swype and didn't have Sense and had a more obvious vibrate.... the first two are fixable, though... so it's tempting. the Rezound's screen was lovely. the Razr's display is only marginally better than the D4, in my opinion.)
Interestingly, i don't really notice my D4 being all that much faster than the D1 was... despite the faster processor and additional RAM. but i don't use it heavily, i guess.
I love the 4G. it's great how youtube videos load with very little fuss.
Anyways, enough for now... hope my opinion helps you decide. basically what i miss most about the D1 is the vibrate (only marginally), the external speaker (also, only marginally), and the screen display (a lot!), and the non-caps-lock keyboard (but the number row on the keyboard makes up for the caps-lock annoyance --so that's a wash).