Mainly because I was looking for more oomph on the bass side. The problem I found was I could boost the bass (through the EQ and a bass boost function), but only at a not so low frequency of 70Hz. Being a bass head that ended up being a bit of a deal breaker. Apparently though, you can buy impedance adapters which fix up the bass, and I probably should have this explored this but just got lazy.
Other than that, I really had few complaints. Really, I didn't find the UI a big deal, I mean sure I would have preferred tactile, and I didn't really like the top-side buttons as they were a bit too tiny/ungainly to use but on the whole once you get used to it, it's just as usable as any other interface. It is a kinda a good mix of both worlds - both touchscreen for quick fast forwarding/cleaner design and a few tactile buttons so you can either adjust volume or track forward/back from your pocket.
Oh and if you're really iffy about the UI, just check it out on youtube.