So, I submitted my first two tracks on Taxi and got one of them reviewed a couple of days ago. And... I had made a completely newbie mistake!
The track was for a cartoon-like cue (looney tunes type thing), and the review was very positive: "Tons of great ideas here, nice comedic moments and accents". Except that... I completely forgot to normalize the level! As a result it was way too soft and couldn't be forwarded to a client. Major face palm!

Oh well, live and learn. Onward to the next one!