Yup, Mustaine never rests from writing, or touring for that matter.
Sudden Death was written for Guitar Hero, but was released single handedly as a single on Itunes and more, thus it classifies as an ''old'' song which was reused on 13.
The production on Death Magnetic is an oddball, it sure does pack a punch, it's really powerful, you can feel it, the problem is in the sound. The songs over-distort, especially the drums when Lars hits the cymbals, which is a side effect of the loudness wars. Death Magnetic is simply TOO loud, and the overdistortion at some points is a side effect of that. The songs are however some of their best written songs ever, I can't get enough of songs like Broken, Beat & Scarred, Cyanide and Judas Kiss.
Also, if you're that new of a Metallica fan, then it's no surprise you don't like the 3 albums you listed. Load and Reload takes some listens, Load is actually a very good, groovy and catchy album, The Outlaw Torn is one of my absolute favorite songs of all time. Reload is a little more odd, it has A LOT of fillers, Fuel is probably the only one you'll dig for quite some time, along with the live version of The Memory Remains. Last but not least, there's not a big chance you'll ever come to like St. Anger, I love it, but the bad production, the lack of guitar solos and especially that damn trash can snare will probably put you off a lot. If you've watched the Some Kind of Monster movie I bet you'll atleast have respect for the album, as it is the biproduct of Metallica's hardest times. Load and Reload were simply albums they made because they felt like making something different than the usual metal, they wanted to make some groovy stuff, and I appreciate that.