I would say the difference is not in the techniques used, but in the end result.
Peggle is just as addictive as World of Warcraft - I've sunk dozens of hours into Peggle and still get a thrill when I beat a challenge or get a full clear. However, my total investment into Peggle is about $60. $20 for Peggle, $20 for Peggle Nights a couple years later, and a further $20-$30 for Peggle DS.
The cost sunk into World of Warcraft though....ooogh.
One provides entertainment and keeps me playing by using the psychological tricks, and gets me to buy the sequel. The other uses the tricks to keep you playing and extracts money from you constantly while you're playing. There is a fee associated with playing WoW or any other MMO, and many of these supposedly "free" online games use those same tricks to get you to buy shiny armor or a pretty hat. That was the concept behind shareware games, and it's how many so-called casual games make up their revenue streams.
An iconic example of this is Gunbound, which ran for years and made a bundle of cash despite being totally free. The number of people I saw with $50-$100 worth of costume jewelry (that didn't even affect the gameplay of the zone I was in) always amazed me. I can only assume the "avatar bonuses ON" zone was even worse in this regard.