To have made the DX90 for a larger battery it would also have to be wider and there you go, wider as they would most likely have used the battery for the S4.
With the advent of the 128gb card, which I have ordered, I will be fine but I wouldn't mind an additional slot either but the DX90 with the dual core CPU will scan even faster than the DX50 so inserting a different card won't be that much of a hassle. Also, they don't use the slide cover over the card, which I find a hassle in the DAP;s that have some sort of cover.
I also have no hiccups with the FW of the DX50 and although the DX90 will be a little different, and there will be some added features on the FW, I look forward to a pretty mature UI.
I think the sound for cost value will be very good. For me, I like the size, shape and feel of the DX50 case. There isn't a great deal of weight but plenty of functionality.