Is it normal for this model to be very hard to push on all types of carpeting and rugs? My wife can't push it at all and I have to struggle with it. to adjust the carpet mode, I just press the appropriate button and that's it, correct?

Yes I believe this is fairly common and has been reported before on here. Have a look at this thread:

http://www.dysonforum.com/viewtopic.php?f=5&t=206

Thank you for pointing that out. I've returned the Dyson, (2nd model I bought and returned this month). Trying a Miele Canister type.