Should I start fresh and what is new?
Right now you should not delete any character made before last April. The older ones are grandfathered in to having all traits available simply for leveling up; newer ones have some prohibitive hurdles to getting at their traits. The system will get revamped again, presumably before or as of HoT launch, but for now you may want your old alts handy, at least until you try out how things have changed and decide if you like it or not.
Beyond that, check out Azhure’s links and perhaps google for other threads asking “what has changed in GW2?” because there have been walls of text answering them. Or just start playing and ask more specific questions as they occur to you, either here or in Map Chat. It’s a chance to discover the game all over again!