What pulled me in is the criss-cross braid down the front. I've never seen one from this decade done like this.
Another interesting thing about it is the fabric. It's damask. Most unusual. The dress description on the Met's website provides some insight:
This is a striking example of how 18th-century fabric was treasured. The textile was probably originally a 1740s dress which was taken apart and then reconfigured into this fashionable dress in the early 1840s. The elongated waist and V-shaped bodice front emphasize the bust and wide shoulders and were key features of the dresses of the period.
Silk damask was incredibly popular throughout the 18th century. What's even more amazing, though, is the incredible condition the silk is in. Which is another clue that it's older than the 1840's. The introduction of analine-based, or synthetic, dyes in the 1840's created a very interesting reaction in silk as it aged. It shatters. That's exactly what it does too, and exactly what it looks like. I've seen shattered silk in person and it's at once amazing and painful. Red and mauve are the worst offenders. Mauve was also the very first synthetically created color.