I loved my OG Evo to death, but I'm so glad I updated. LTEvo is a true successor. My one and only complaint was the lack of a menu button, but that was easily fixed by flashing a zip found here. LTEvo is just so darn fast! It's so fast that without doing a speed test I really can't tell if mods help. It's like trying to tell the difference between super fast and super duper fast. I've hardly ever seen the word buffering or the loading icon.
And I'm so happy with the giant screen. After you get rid of the three dot on-screen menu button you have the full screen available and it's beautiful.
Now to your questions. ROM development has been great. LTEvo has been it for only a couple weeks and there are already enough great ROMs to keep you entertained for just as long. Plus all of the mods and tweaks, it's wonderful. As far as I can tell S-Off isn't even necessary any more. You can do everything with S-On after you unlock bootloader. Not many kernels yet, HTC hadn't released the source code for us yet. A2sd isn't necessary either anymore. With the ridiculous amount of internal storage you're all good. They give you more space than people ever partition for a2sd.
I've never used gnex so I can't really weigh in there. But I must say if you're thing about getting LTEvo, go get it right this second! Don't even look back. When I use my parents' OG Evos they feel small, thick and slow now. LTEvo is big, slim, fast and OG so beautiful. Okay, done ranting now. Go get LTEvo.