I'm guilty of this i must admit, my problem for years was in the name 'ultra' like they're meant to be something special and i remember my puzzlement (we're going back beyond 3rd ed now) when i found out that there were other chapters with an awful lot more character. btw, Have we had the 'they're space romans' discussion yet?
The four biggies as far as spacemarines are concerned have traditionally been the ultramarines, space wolves, dark angels and blood angels. In SM terms that translates as the romans, barbarians, priests (not the kiddy fiddling type though) and the mentally unstable. I mean, which of those is going to be more fun to collect? In those terms the UM definately show connatations of imperialism in it's purest form and quickly loose their appeal.
Anyone that's been collecting over a number of years must have seen GW trying to react to the backlash against the UM. It's the UM that have had the most famous victory over tyranids, defeating the supposedly indestructable, through to that mental UM chaplain fighting off a bloodthirser in DOW winter assault. It's PR
gone mad i tell you, just imagine what the dark eldar could do with Guillman's agent!
There certainly is a 'default choice' value to an UM army, particularly for newer players but over some years i've developed a respect for the more experienced players that actively choose to play the Ultramarines. It's taking the idea of a SM army back to it's core, cutting out the waffle, there's possibly even an 'ironic cool' to it. They're still pretty boys though.