first up, i wanted to say that you’re probably only going to get biased responses from the people who troll the forum 24/7 to complain about how bad the game is (while they continue to maintain multiple accounts, go figure!) 
heirlooms have had quite a considerable change, they no longer have XP baked into them.
so if that was one of the things which was keeping you away from the original account, it probably won’t be as much of an issue for you now.
if you’ve got years of content to catch up on, you’ll have no problem keeping yourself entertained without the current expansion.
…if you feel like you might stick around for a while, then it won’t hurt to buy the expansion while it’s on sale.