I've recently switched and there are a few things you need to consider IMO.
First, how much have you got invested in software that needs to be replaced. By the time you've got Office, Acrobat, Photoshop etc in the Mac versions, it can cost quite a bit. For the PC you will already have the discs in a drawer somewhere. So seriously work out what software you will need and how much it will cost. If you want to run Windows on the Mac (works quite well) remember to cost in Parallels. I use it because there are just some programs with no Mac equivalent e.g. Poptray
How much of your old data do you need? If you can start afresh and leave it where it is that's fine but if not think about what you will need to transfer over. ITunes transfer is a right pain for example and if you don't transfer it, the new iTunes will want to wipe your iPad/iPhone/iPod. Ditto moving over emails, browser settings, saved passwords etc. With a PC the swap is much simpler because you just copy the files over to the same location on the new machine or with Acronis you can even clone the old machine onto the new hardware.
How much do you like to fiddle with the system? The basic transfer to a Mac is quite easy but if you do more than that do not underestimate the learning curve for all the tacit knowledge you have built up especially if you need to troubleshoot. e.g working out what the equivalent is to Ctrl-Alt-Del is to look at what processes are running. The Mac is also a lot more locked down against customising (as evidenced in the extreme by the iPad/jail breaking tussle) which is the price you pay for things tending to trip over less.
My top tip though is to do what I did and get the Mac on their 14 day returns policy, get the trial downloads of Parallels, Office etc and see if you get on with it. If not return it. For me it was a choice of a 13" Sony Vaio or MacBook Air. After 14 days of serious thinking, I kept the Air but it was after carefully weighing up of the pluses and minuses and though I like the MBA, it does frustrate me at times when I don't know how to do something that would have taken a few seconds and have to start reading up instead.
Good points and considerations. Well done.
With regard to the software replacements I'd add a couple of things. Most software manufacturers have Mac versions today and they more often than not cost less, sometimes considerably less than the equivalent for Windows. MS Office for example is about half the price.
Another consideration is that there are often compatible programs such as Apple's Pages (Word), Numbers (Excel), etc. which, again, cost far less than their counterparts.
Also with the Mac App Store instead of being forced to buy a suite of programs, some of which you may never use, you can buy almost everything ala carte.
The other consideration is that there is no "tweaking" to get things working...no worries about drivers being compatible, fiddling with the registry, problems with virus software conflicts, etc. Again, almost without fail, software on a Mac "just works". Updates are generally one click...done.
So yes, the one-time replacement of some software is in order, but that's often no different than it is with PC's as software goes out of date, updates have to be purchased, etc. I can't remember the last time I paid for a company's software update on our Macs. (I don't think I ever have?)
FWIW, there's no need to buy Parallels to run Windows on a Mac as it comes with Bootcamp which works perfectly. If you want to run some Windows programs in your Mac environment (or just don't want to reboot into Windows) Parallels is a great program (I used to use VMWare Fusion, but really like Parallels now).
So although like buying a new car you may have to buy new floor mats, the overall, long-term cost of maintaining a Mac has been less, a good deal less than that of a PC in my experience. YMMV of course.
With respect to "fiddling" with a system...having done that for years...mostly because some software or hardware was not performing properly, using a Mac is pure heaven. It's not without faults once in a very long while (and usually nothing that a quick reboot won't solve), but I have saved hours, days, probably months of time simply not having to deal with computer problems. Crashes and freezes are a thing of the past. My wife has yet to call me into her office to fix anything on her Mac. Woo hoo!
I don't know about anyone else, but I consider my time valuable and I can't calculate how much time I've saved not having to "fiddle" with anything since switching to a Mac.
As far as reading up on how to do something, I haven't really had to do a lot of that as Windows has moved toward Mac in a lot of respects, but when I do I find it enjoyable. I subscribe to MacWorld and Mac|Life and really look forward to learning the finer points of OS X. I've even created an Apple Developer account just so I can have a play with OS X 10.7 Lion...what great fun! I guess that's one of the differences for me. PC's ended up just being work and Mac's on the other hand handle all of the workload, plus they are downright fun to use.
So no argument about some of the considerations when switching to a Mac, all very good points and well worth thinking about. Been there, done that for sure. When I look back on it, I would do it all over again in a heartbeat!