For someone who wants to keep the machine for a LONG time though...
Target Display mode would allow for a long life as an external display and TB3 will likely enable things like real external GPU upgrades....
...and USB-C?
It would be VERY Apple like to go "all USB-C ports" immediately at some point not too far off and leave everyone using adapters. They *love* to do the "here's the new thing - deal with it" routine and the MacBook has shown us they may do just that w/ USB-C
Gah! That's what scares me, to be honest. I'd ideally like to make this next purchase last as long as my 2010 MBP has, so it would be a real kick to gut if, buying an imac now, they come out with a model next year that triggers a change in the ports that everything uses.
Blah, first-world problems, as they say.