Learning to sing with a good tone can be tough. I'm no singer, but I can speak from the perspective as someone who has had to take classes in classical sight-singing.
It sounds like the timbre of your voice would work well for the Coldplay style, and your diction sounds pretty good based off of the Viva La Vida recording and the other two from your previous thread. However, it's still falling a tad flat as a performance. Don't try to create more drama in the song though, start with the smaller, fundamental things and move up from there.
First, have you worked out the melody on keyboard/guitar/midi/whatever? Try to tune each note, work on the transitions between notes--I find when sight singing (especially when I hit passages with notes that aren't in the major scale) you can easily slide out of tune when moving between some intervals or scale patterns. If you're able to prepare ahead of the recording session or performance--save yourself the worry and internalize the pitches beforehand. You mentioned in your previous thread that you feel like you have a good amount of control over pitch, I can see that here, but it sometimes sounds like you are trying to do a bit too much with embellishments like slides and ornaments. For example, in your recording of Re: Your Brains, you had a lot of portamento going on, but if you listen to Jonathan Coulton sing it he actually doesn't do a whole lot with the melody outside of dynamic phrasing. People will value someone who's a little more straight forward and dead-center on the pitch than someone who tries to do a lot of fancy stuff but in the processes loses the center of pitch.
Another thing that Dave J mentioned in your previous thread is air support. Try an exercise in which you do the cliche meditation noise "ah-oom", except try to make it deep and sonorous like James Earl Jones playing Darth Vader, or like a baritone opera singer. It will sound really cheesy, but trust me if you try to mirror in singing what you're physically doing with your lungs and diaphragm to produce that noise, then when you sing in your real range and style then it will be fuller and any nasally characteristics of the sound will diminish. This is something I'm always struggling with as well, and usually sitting up straighter and thinking more actively about my rib cage and air support really sweetens my tone as soon as I do so.
The next thing to work on would be your phrasing--the best example I can cite is the line "But that was when I ruled the world". It sounded as though you took a breath between "I" and "ruled", or if you didn't then at least the sound was stopped. Listen to the recording and you'll hear that he carries the phrase through to the end of "world" then is able to take a breath during the instrumental break. This issue will become less apparent as your other skills improve, so once you're comfortable with pitch and air support, don't forget to spend some time on phrasing.
A really good book for learning to sing is Melodia:
http://www.sheetmusicplus.com/title/Melodia/1910955
It's basically a gigantic collection of melodies composed in such a way that you get used to singing in all the different keys with varying levels of chromaticism. Prepare and practice some of them, and then try to just sight sing others.
