The fundamental problem with NAND-based solid-state drives is that they use NAND flash memory--the same stuff that you find in USB flash drives, media cards, etc.
The advantages of NAND is that NAND is both ubiquitous and cheap. There are scads of vendors who already make flash-memory products, and all they need to do to make SSDs are to slap together a PCB with some NAND chips, a SATA 3Gb/s interface, a controller (usually incorporating some sort of wear-leveling algorithm) and a bit of cache.
The disadvantages of NAND include limited read/write cycles (typically ~10K for multi-level cell drives) and the fact that writing new data to a block involves copying the whole block to cache, erasing it, modifying it in cache, and rewriting it.
This isn't a problem if you're writing to blank sectors. But if you're writing, say, 4KB of data to a 512KB block that previously contained part of a larger file, you have to copy the whole 512KB block to cache, edit it to include the 4KB of data, erase the block, and rewrite it from cache. Multiply this by a large sequence of random writes, and of course you'll see some slowdown.
SSDs will always have this problem to some degree as long as they use the same NAND flash architecture as any other flash media. For SSDs to really effectively compete with magnetic media they need to start from scratch.
Of course, then we wouldn't have the SSD explosion we see today, which is made possible by the low cost and high availability of NAND flash chips.
