Apart from the fact that it looks beautiful and has no nasty wires (wives hate wires don't they!)
Tell her that she can now surf the web and use her credit card in the confidence that there will be no spyware/keyloggers taking a copy of her CC number and sending it across the net. Reports say that 25% of all PC's with spyware and AV s/w STILL have some form of malware - that's a very worrying thing for a husband to have to deal with!!
Plus when she actually uses it for a few weeks and starts to do things that she simply couldn't before she'll realise it's pretty awesome. Also, make sure she's there for the 'welcome' video that runs on first boot up - that always impresses - makes her feel 'part of it' too..
It'll take a while before she realises that it IS different from a PC - her own personal user experience will make her realise it's a very smart move.
Don't forget the re-sale value and the fact that this little beauty will last years and still be worth hundreds more pounds when you come to sell it than a junky PC.
There really are a ton of genuinely valid reasons to have made a wise investment in a Mac than merely buying another crappy Vista PC - I think she'll realise that if she just gives it a chance...
Anyway, good luck and enjoy your new Mac - you ordered a great spec iMac...Your going to love it!