I came from a Windows Mobile, HTC Touch Diamond

But I definitely see your point.
Facts are facts - "It was quick, browsed better and to be honest was fairly intuitive to use", very true - might I add, with the latest round of CPU's in them (ie. 3GS vs 3G), they're VERY quick.
Because I haven't had any long term experience with a 3GS I can only speak on the perspective from the latest generation iPod Touch in 32GB edition.
There's some minor hardware differences between the two, but they function almost identically, minus the phone.
The iPhone was built for the consumer, where as the Storm2 (well, any BlackBerry) was built for the enterprise user. This doesn't mean they don't work both ways though, as is clear between the two, the Storm2 with it's touchscreen is aimed at better fitting the consumer where as the iPhone also has email facilities as you'd expect from an enterprise based phone.
With that said, it's obvious that the iPhone will suit most people purely because, Apple have a target market and they've hit the sweet spot. Same goes for RIM, only there's a difference.
The iPhone aims for simplicity, that's what the consumer wants, hence why email, browser, GUI and the "App Store" are all fast and easy to use. The iPhone if you ask me is one of the best phones out of the box for the average person. It just works.
Now comes the problems of the iPhone; security, enterprise integration and maybe even reliability. Naturally RIM are the masters at this and you can find these features in the Storm2. But that doesn't necessarily make the Storm2 superior, it really comes down to what you want. Certain features in each phone may be superior to each other from a facts point of view (eg. iPhone is faster than the Storm2 with general use) but it comes down to which features between the two are superior to you.
It pseudo multi-tasks. Reminds me of my Palm days. You can play music while doing other tasks, also have instant messenger running and be installing an application over the air via App Store all at once. There's no real disadvantage of not being able to multitask, unless you specifically need it for whatever reason. Calls will come through like normal, even if you're in the middle of something.
Not sure but the iPhone 3GS is roughly the same thickness as the Storm2. It's a good 30grams lighter though.
I've heard they drop calls, but can't confirm it's a real issue, may also be carrier related among other variables. All phones drop calls, it's a hard one to pick unless it drops virtually every second call.
Depends on what you use the iPhone/BlackBerry for, data plans are getting cheaper and cheaper with more quota per month. A lot of applications on both the iPhone and BlackBerry use an internet connection for whatever reason (maybe a weather update, Facebook, any instant messenger, Google Maps etc.) so depending on your usage, unlimited internet sometimes isn't such a bad thing, especially with Vodafone's latest price offering of $10.
Only problem is this, BIS dropouts and slow data transfer. It's only slow because everything goes through a server of some sort at RIM's end then to your phone. That's why generally, things happen faster on the iPhone when it comes to applications accessing the net.