Letter columns still exist. Several comics have them. Avengers Academy, Daredevil, Journey Into Mystery, Uncanny X-Men, just to name a few. Often, the letters columns are actually a lot of fun. Actually, a recent issue of Daredevil had a letter from someone volunteering to do some art for the book. Some dude by the name of Mike Allred. The artist for X-Statix. So that was cool. Waid suggested maybe Allred could do an alternate cover, so I'm hoping something comes of that.
X-Men X-Pert, my blog where I talk about X-Men comics.I'm actually reading that era of Spidey right now via the Essentials. I actually totally understand Kurt's comments there. That era....isn't one of my favorites. It's ok, but I do feel that it'd gotten stale and played out.
As for modern letter pages, Avengers Academy is notable because Christos Gage answers all the letters himself and has spent extensive time discussing the topic of gay characters and politics (one writer objected to a brief appearance of the Purifiers) with readers. Uncanny and Wolverine and the X-Men are notable (and often extremely entertaining) because they are written in character by a different X-Men character every issue, completely tongue-in-cheek.
Ji M combines those things. Gillen replies to the letetr himself, but in a very tongue-in-cheek manner. And even those make me laugh my ass off. Journey Into Mystery is one of the funniest comics ever, while also being one of the smartest. And it also had one of the saddest issues I've read of any series.
It's just an implausibly good book in general.
Anyway, Fantastic Four, FF, Punisher and Amazing Spider-Man have letter pages, too, and I'm sure I've forgotten others. Letter pages have actually made quite a comeback.
X-Men X-Pert, my blog where I talk about X-Men comics.

I had to share this somewhere because it was amazing.
I was just reading Spider-Man #188 (from way back in '79) and there was a very long, very angry letter in the letter column written by none other than Kurt Busiek. He railed against the entire Gerry Conway and Len Wein runs on ASM, and lamented that Marv Wolfman (Wein's successor) was no better. He was angry that Mary jane rejected Spider-Man's proposal in ASM #182 because "the idea of a married Peter, bouncing between a teacher's assistant job, the Bugle, home strife, and the usual plethora of super-villains, really appealed to me." Most of all he longed for a return to the days of Stan Lee.
My favorite bit: "Speculating about what will happen next in SPIDER-MAN is like wondering if the good guys will win on your seventh viewing of STAR WARS. Spidey has degenerated into a red-and-blue Mego doll; other than the fact that he's a nice package, he's the same as most other super-heroes around. He is no longer in the vanguard."
It's always fun to see future comic writers in letter columns. Kinda makes me wish that letter columns were still a thing.