I find that starting a new character after you already have a "main" guardian that you've had since the beta is truly THE MOST DIFFICULT THING...period.
I've been a hunter since beta and decided I want a warlock for Tlaloc (I -blam!-ing love scout rifles).
I jumped to level 25 and made it to 27 before that "ugh" feeling began to set in. In the back of my mind I know I have a fully kitted out hunter ready to go, but if I want Tlaloc I've gotta grind through this character to do so.
It's odd really, in other games I've had no problem making completely new characters and starting over per se, but with Destiny, it feels like a death sentence.
I'm not going to bash Destiny and say it's not fun etc etc, but something about making a new character when you already know you have a kitted out character is just...tough.
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Lvl up just using pvp and its a lot of fun especially if you are using a class/subclass you never played before. It's also much faster. On my hunter I literally played only one story mission just to get to the tower and the rest was pvp.