If the emmc is smart, writes within a 256k block would be fast... Problem is filesystems like ext3 sprinkle data far apart, and have layouts optimized to avoid fragmentation.

Basically filesystems designed for harddrives do all the heavy work when writing, they place various datastructures all over the harddrive in order to avoid fragmentation and so that when it comes time to read or find stuff, the data it needs is close together and not fragmented.
The idea is to take the performance hit when writing, since you'll probably read the data more than once anyway.

This on harddrives where sequential writes and reads are fast, and both random writing and random reading is slow.
On emmc, sequential writes and reads are fast. No difference there. The difference is in random writes and reads. Random writes are REALLY slow. Painfully slow. Random reads, however, are pretty fast.
Thus, the complicated and thought out optimizations that make regular filesystems fast on harddrives end up hitting the weakest point of emmc storage, hitting it hard.

The nilfs2 filesystem is almost like a gigantic journal. Data is written out mostly sequentially. Data is rarely replaced, new data is instead written, with a newer version number. This was the goal of nilfs2, to be able to jump back in the journal/log and restore the filesystem to a previous state. As it happens this ends up in nilfs2 being very slow on harddrives, because the data is all over the place when it's time to read or find data. On emmc these random reads aren't all that bad, and nilfs2 generates alot less random writes. Avoiding the biggest weakness of emmc makes it more than 10 times faster in my test.

LogFS is actually designed with flash based storage in mind, so it will be interesting to see how it performs.