Helen writes about why I-mode failed in the UK. I'd add two reasons to her list:

1. Handsets were rare and often rubbish. The little NEC device is a classic example of what I wrote about in my Cocoon review: lovely on the outside, absolutely appalling on the inside. I think it had 3 different ways to do messaging, all with different menu structures: a canonical example of Asian handset manufacturers failing to do UIs appropriate for Europe. Other handsets were better, but the NEC one was one of two at launch: first impressions last.

2. O2 didn't commit fully to I-mode, promoting I-mode alongside O2 Active without any clarity on why customers might choose one over the other (or opt to migrate). The end result was that whilst O2s customer base is massively predisposed towards mobile content, its I-mode customer base was tiny (we've run services on the portals of both and the difference was staggering). I don't know of any other operators who ran I-mode alongside competing mobile data services, does anyone else?
   

A shame in a way - it would've been nice, as someone working at the content end of the industry, to see a service based on more favourable revenue shares work out - but as Helen says: it was too late.