Weather sealing, larger buffer, faster card slot (with side access), a more substantial body with battery grip option, a few extra dials, a PC flash connector, a larger EVF with dual view, exchangeable eye-caps, optional handgrips in 3 different sizes, USB tethering and 3 custom white balance settings.
Of course, there are also plenty of features that are exclusive to the X-T10, so...
Just picked up a silver X-T10 yesterday and it is a superb little camera with the emphasis on little!. As has been said before, it all depends upon how you intend to use your camera and it is worthwhile checking the comparisons on the net to see if the additional cost of the X-T1 is justifiable.
I have the X-T1 and only bought the X-T10 because my trusty X-E1 was being kept in the cupboard so I took advantage of the Fuji UK trade-in scheme.
I love the X-T1 for all the reasons Rico pointed out but, had I not already purchased it, I would really have to think hard before coughing up the price difference. IQ is exactly the same, I am not particularly interested in the weatherproofing and the only thing that would sway me would be the larger buffer size on the X-T1. But the price difference would go a long way to a lovely prime lens!
Had Fuji not introduced V.4 firmware for the X-T1 it would have been a no-brainer not to have gone straight to the X-T10. As the size, and grip, between the two bodies is so different I would advise trying them both side by side as the X-T10 is appreciably smaller.
I have the X-T10. Tried the X-T1 when it first came out but didn't really get along with it. Have had a number of Fujis in the past and bonded with the X-E1 which I traded in. I'm getting a bit more fun factor with the X-T10, focus is improved from the X-E1. The only caveat I would add is decide which lenses you will be mostly likely to use. Whilst the 56mm 1.2 is quite chunky, it does get heavier after a while and more uncomfortable to grip.
Since none of my current Fuji lenses have weather resistance and my funds were a bit limited this time around, the X-T10 body-only made much more sense for me. If I went for the X-T1, I'd be constantly tempted to add some lenses with WR. So far, the X-T10 has been a lot of fun. No regrets.
Hey Rico (or anyone else who knows)... On the X-T10, can you assign the 4-way buttons on the back plate to instantly move the AF point, like you can with the X-T1? Because I've gotten completely hooked on that.