I think we will get a better opinion what the value of DDI is when it goes open beta, but still...$120 a year does seem a little steep for what it currently appears to be. Granted having access to all the manuals and future splat-books, the service could "pay" for itself in the short-term. It's just that software as a service doesn't fair that well here in the USA...people like to own stuff, and not be "taxed" when using it.
Now on a tangent, I think WoTC is really missing an opportunity with DDI/Gleemax. I firmly believe that one of the reasons why DnD has been a success is because of community creativity. Remember the old AD&D Slavers series (A1-4) from TSR? If I recall it was a RPGA tourney from Gen Con back in the day. Look at some of the quality of products from third-party publishers...some of it is pretty damn impressive. On the digital front, NWN had a huge Mod community publishing adventures for that program. The point being, if WoTC wants to charge $120/yr for software as a service, they need to show the subscriber the value of that subscription....or as they say ROI , Return Of Investment.
WoTC mentioned micro-transactions, why not micro-transactions for subscribers? Got a kickass adventure? No problem, as a paying subscriber I use DDI to create the module, then upload it to Gleemax (or wherever). This module is a PDF that can be purchased by anyone (no subscrition to DDI needed), for let's say $10.00. WoTC gets $5.00 for each sale, and the author get's $5.00 for each sale. Now we are talking about empowering the DnD community...if only 24 people purchase said adventure module, that subscriber's DDI subscription has paid for itself. ROI !
Ultimately...this is how you sell software as a service.