I will take a shot and give you a vague answer to your vague question.
There are so many innovations in grilling that it is like asking which is your favorite child. My short list would include rotisserie, Weber’s Touch N Go ignition for the Performer, and better grease management systems in gas grills. Most useful, necessary and irreplaceable accessory is without a doubt quality tongs. Close behind that are a charcoal chimney, an instant read thermometer and a timer.
I am not too hung up on one particular grill or gas vs. charcoal. To me my grills are like tools in a toolbox. I use the best one for what I am cooking. Having a variety of grills available helps offset the disadvantages and weaknesses of any particular grill. If I could only have one, it would probably be a Weber kettle grill.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill