Do people not like Fuel? I know it gets made fun of on the internet but it's hard to tell if people actually like the song or not when music becomes a meme. Anyway I think it's a pretty cool song, despite sounding absolutely nothing like Metallica.
1: My favorites are Call of Ktulu, Ride the Lightning, Master of Puppets, Battery, Orion, Damage Inc., Blackened, most anything from those three albums really. My least favorites are Thing That Should Not Be, Nothing Else Matters, almost anything from Load/Reload/St. Anger and The Day That Never Comes. Metallica is a very hit or miss band for me. There are songs that I absolutely love and then there are songs that I can't stand. Very little middle ground for me.
2: I don't like it. The Black Album has its moments and I don't mind that they went in a more commercial direction, they should make whatever music they feel like making. But god, those albums are bad. I don't have a problem with commercial sounding music but I don't think they're good at it. The Black Album is loaded with filler, Load/Reload could be combined and still wouldn't be an even decent album, and St. Anger is pretty wtf. I think Death Magnetic is pretty solid though, I'd probably place it at #4. Which honestly isn't really saying much though since I think there's a huge drop in quality after my top three. I'm not gonna hate on Lulu though because I think they achieved exactly what they were aiming for. I think they were trying to make an album that would irk both Lou Reed's and Metallica's fan base, and they did an amazing job. One of the best trolls in recent music history. Plus it's not that bad. I'd take it over anything Metallica has done since AJFA (with the exception of Death Magnetic) any day.
3: The by request system is cool but it's not really necessary for Metallica. They're already really great at writing their own setlists. You get all the awesome songs that you'd expect to hear at a Metallica concert, some recent stuff, and then a couple of rarities thrown in for good measure. They don't need requests because their setlists are already awesome. Same with Dream Theater, there aren't really any songs that fans are dying to be brought back because they change it up so much. Especially lately they seem to like dusting off old classics so we'll probably see a lot more of that. Iron Maiden could benefit though. Do the regular Maiden setlist and then throw in a few "by request" songs. You could get some awesome results.
4: Hopefully something in the vein of Death Magnetic but with decent production and more concise songwriting. Besides The Call of Ktulu, I can't think of a single Metallica song that benefited from being long. It's wholly unnecessary, they could really trim down some stuff. In fact, the biggest reason why I don't go back to DM songs much despite really liking the album is that they're so needlessly long and I don't want to sit through 9 minutes of something when all my favorite parts are surrounded by filler sections.