A problem with the guitar isn't just the balance, but also how much space it's occupying frequency-wise. There's lots of pick noise and the guitar is generally quite bright. That's not necessarily bad, but it can distract from the vocals if there's too much going on at once. I'd be tempted to put a little bit of fast compression on the guitar to take the edge off the pick noise, for starters. The guitar is also a bit echoey, which doesn't sit well with the fairly subtle reverb on the vocals. If you can lose (or reduce) that, I think the guitar sound would be much improved. In general though I think the guitar playing itself fits the song well.

Vocals mostly sound nice, but you really do need a pop shield. It sounds like you've got a little too much compression (possibly in an attempt to get rid of the pops?) because there's not so much in the way of dynamics.

Don't forget that you don't just have to leave the faders in one place throughout! Bring the guitar down by 3dB during the verses to leave space for the vocals, and bring it back up again between verses. Similarly, there are a few places the vocals drop back a little (off the mic, maybe?) - you can re-record that line again, maybe, but to some extent you can compensate by boosting the level slightly where it happens. And better to do that with the levels than just slapping compression across the whole thing.

I didn't think the fiddle sound worked - it just didn't sound like a fiddle should, for me. Generally, I think that's something you could improve on with a little more experimentation with different mics, and relative locations of mic and fiddler.

Jim's point on MP3 is just that we can't hear all the detail that we'd hear on a CD. If you recorded at 44kHz, you're fine. Moving to 48kHz will make no difference (and in fact could itself be the source of noise when it's converted back to 44kHz for putting on CD). It's worth using 24-bit though - if you're recording at 16-bit, move up to 24-bit otherwise the details of the recording will be lost. (In case you don't know, the problem is that you have to leave headroom when recording, but that headroom means most of your sound comes from less bits, giving lower resolution. 24-bit still gives you CD-quality with 48dB of headroom.) I'm actually wondering whether that's the problem with the fiddle sound.

For making things louder, beware just slapping that mastering limiter across a ready-made mix - it won't sound right. There's a great article in SoundOnSound this month about mix-bus compression and its relationship to compression/limiting on individual channels and mastering.

Moving on from the technical stuff, generally I think the arrangement could use a little work. The fiddle dropping in and out has already been mentioned. In general, what's the fiddle's purpose? It's not emphasising important lines or important changes of feeling, it's not providing variation between verses, and it's not providing a continuous background to the song. Making it serve the song will give it some raison d'etre.

Also on the vocals. This is a very well-known song, so I think you need to make this personal to you, instead of just one of a zillion adequate-but-nothing-special covers. It's a story song, so tell us the story like you mean it and really grab our attention. At the moment I don't feel empathy with what you're telling me because it doesn't sound like you do either, so it just sort of washes over me without making an impact. Sure, it's a gentle song, but your voice and playing (and the quality you've extracted from that recording setup!) deserve a lot more than just being pleasant background music! If you know how your character is feeling in each line or verse, that'll inform your singing and will probably then spill over into the arrangement as a whole.

Graham.