Well, I did it, I switched my N900 for my iPhone, it was fairly simple to export all the contacts from my N900 and import them into my iPhone 4S (surprisingly).

As a preface; I have owned the 1st generation iPhone for 4 days after returning it very displeased. I will now present my findings in bullet point format:

Psychical:

• The lack of a psychical QWERTY keyboard is defiantly made up by Siri; incredibly useful voice recognition, also it has spell check thus I am almost as proficient as I am on the N900, and I am getting better everyday.

Lacking Features:

• One of the the things I miss the most is the auto speakerphone when you take the headset away from your ear that was on the N900.
• I don't like not having a basic file structure available to me.
• I do not like that I cannot access "mass storage mode" and directly place ringtones and files of that nature onto my iPhone

On Multitasking:

• Same basic concept as Maemo 5, only displayed very poorly, however Jailbreaking can fix that and make it a bit more proper.

On iOS 5.1.1

• Very confusing to someone like me, when things are "dumbed down" it is harder to understand, it might be the opposite for people with less technical experience.

On 'Apps':

• Very overwhelming; on the Maemo nearly all applications were quality applications. Even from devel and testing. The mindset behind the N900 was, "How many features can I put into this thing?" Whereas on the iPhone the mindset is, "Which one should I go with, what's the best?"
• Makes life so much easier, I deleted every restaurant I dine at often out of my phone, I can now just use Siri and say, "Where can I get pizza?" and my favorite pizza places will pop up. Same concept with banking applications, movie applications, etc.

Lack of Flash:

• I have not had a problem with it yet, the YouTube player surprisingly plays every video I watch perfectly, there's an App for that.

On Social Media Integration into iOS 5.1.1:

• Surprisingly lacking, especially in comparison to Maemo 5

My finding so far is that the unjailbroken iPhone 4S can mostly do everything that Maemo 5 can (...mostly...), only much faster and much more efficiently. It can also do a whole lot that Maemo 5 can't. There is a lack of soul to the device though, almost devoid of all personalization - a 1984 feeling to it. I will be updating this thread in the coming weeks.