I took about a 15-year hiatus (not counting a brief binge of car modeling 10 years ago), and I can say the single biggest change is, of course, the Internet.
When I was building planes and armor in the past, I had FSM, Shep Paine's books and the History Channel (back when it showed more history than reality TV - still mad about that). Now, I can sit on my couch and watch YouTube videos on how to do various techniques, I can talk to everyone on here, read blogs and follow Facebook pages dedicated to modeling, and show my builds to people who understand the work that went into them.
I remember using the Internet one time for model research as a kid - building a German half track with a canon and trying to find out if the crew wore panzer wraps or infantry uniforms. I don't think I ever got the answer, but if I were to ask now, I am sure there would be an answer within a day.
The other main difference is my willingness to spend on some of the aftermarket parts (pre-painted PE sets are just amazing) and to spend a little extra on kits themselves. I remember when the Tamiya Bf-109E3 came out, and I really wanted it, but couldn't convince my mom to buy it for me and didn't want to save for weeks to get it.
Now it looks like this on my shelf:
Of course the issue is still there as I eye those gorgeous Tamiya 1/32 Corsairs and Mustangs...
Finally, and maybe this is more a function of me not being up on things when I was 14, but it seems like airbrushing techniques have really advanced. Preshading, color modulation and especially Alclad lacquers have made a big difference to the models I've built since October and the ones I built as a kid.
Welcome back!
-BD-