My main character, the one I've done all quests on and discovered all points of interest, found all skyshards, learned all crafting, etc. is now obsolete thanks to Subclassing. The builds that I need to stay competitive are no longer possible on my main class, so I must reroll another character and start fresh. Either that or buy skill lines from the crown store and switch characters anytime I need something crafted. You get the idea.
It's also been said before but Subclassing has distilled combat down to maybe 5 skill lines for the serious high-end competitive gameplay.
"oh wow, you're playing XYZ... how original" etc