You didn't mention the gear you used... Mics? Preamps? Audio I/O?

a few initial thoughts...

Lead vocals are lacking "sparkle". What mic did you use? It sounds like a dynamic... and not that this is a bad thing - thousands of hit vocal tracks have been recorded using dynamic mics like Shure SM7's, EV RE20's, even 57's and 58's ... but yours is lacking the sparkle and "air" that I would prefer to hear. If you are using a condenser, well, then it really doesn't sound like one... you've definitely attenuated the hi freq's. The good news is, the vocals don't sound harsh - which is a refreshing change of pace to my ears, when so many lead vocal tracks these days are brittle and nasty sounding...so you're okay there - but ... you need to bring some tonal life and presence to the vocals.

Personal taste... I would open up the drums with some tight convoluted reverb... they are fairly dead sounding.

Did you use a stereo coincidental pair over the drums? If so, widen them out a bit with your pan values in the mix. If not, you can try to open them up with some early reflection delay or, as mentioned above, a good convolution reverb - look at room sounds, and not anything big like halls, plates or chambers... and, don't use a lot of it, either... just enough to liven them up. A little ambiance goes a long way. You don't want to swamp the tracks to the point of smearing and killing your definition. ;)

Oh! And don't mix them in solo mode! Work on them while monitoring the other fundamental tracks in the mix. You can spend hours mixing in solo mode and get an instrument sounding good to and of itself, but once you hear it in relation to the rest of the tracks, they could sound terrible. (IMHO of course).

So:

Add some sparkle/presence to your lead vocals - you've already got warmth and "weight", but you need some presence and clarity on the top end.

Hi-pass everything on all tracks - except bass and kick - from around 150hz or so and down. For tracks like guitars, you could even HPF a bit higher -making your corner frequency up around 250Hz - 300Hz.

Right now, it sounds like you have a lot of energy in the lower mid-range that is stacking up - but not doing anything beneficial. Put it this way... there's no point in adding anything much below 100Hz or so on a vocal track, because the average human singing voice doesn't present frequencies that low to begin with. To add anything lower than 100 hz or so is futile, because there's nothing down there to tonally effect in frequency, and all you'll end up getting are artifacts in low frequency energy from other surrounding sources - HVAC, Trucks going by, etc.

And... don't discount the possibility that this "mud" could very well be - at least partially - caused by that tape saturation plug you are using.
(I'm not really a fan of tape emulation plugs, having come up in the age of analog tape, I've yet to hear a tape emulation plug in that sounds anything like tape, so take my suggestion with a grain of salt.)

Did you use any limiting anywhere? Perhaps limiting on a discreet track level of any kind? If you did, what type and to what degree? If you used compression, what were your values in terms of ratios, release times and thresholds?

Overall, the mix sounds muddy to me, lacking clarity and sparkle. Generally, this is caused by lower mid-range values. Look to your lower mids overall, (300Hz - 500 Hz) and see where each track sits in relation.

I like the song, though. :)

Thanks for the constructive criticism about the mix and the kind words about the song, Donny! Sorry for not including the gear I used, but thanks for trying to help me out regardless. All the vocals were recorded with an SM7B through a Great River ME-1NV. The guitars were recorded direct in via a Pod X3 Pro, but the feedback at the beginning and end of song are from a Marshall Valvestate amp, also recorded with the SM7B. The bass and drums are both virtual instruments. The drums were done with Abbey Road Modern Drums. The bass is the Orange Tree Rickenbacker bass (we usually record bass live, but we were digging the tone of this so we decided to keep it in there). Finally, my audio interface is an Apogee Duet and I used Logic Pro 8 to mix.

I definitely hear what you're saying about the lack of sparkle in the vocals and the "deadness" of the drums, so I'll try to remedy those issues.

I tend to overuse saturation plug-in, so I am assuming that it's at least contributing to the low-mid range build up. I do have hi-pass filter on most of the tracks, but I think I can be less judicious with it.

Like I said before, the drums are a virtual kit, but I bounced out the MIDI "performance" as a multitrack, though, which is what is in this song. There is a stereo overhead track and stereo room mic track that I can use to open the drums up more.

There's actually no limiting or compression on this track. I have nothing against compressors or limiters, I just wanted to try automating levels via my DAW instead to see how doing that to a mix would compare to my older mixes of other songs (which usually employed a lot of compression). Would compression or limiting be helpful for this mix in some way?

Thanks again for the feedback, I really appreciate it. I hear what you're saying and am definitely going to incorporate your advice.