I dont have any regrets in vanilla WoW.

I never finished naxx, but that whole situation was out of my control (server drama). basically I was in the top guild. the other faction wouldn't open the aq gates while we are progressing naxx, <insert drama>, guild implodes.

an unfortunate situation but its water off my back. I can't say I care very much about pve achievements. pretty typical pvp player attitude I guess.

my regrets are more along the lines of not quitting soon enough, installing WoD, things like that.

That i did not pushed my PvP Rank more.

I helped a lot of people in my guild with low level Dungeons or farming the T0 sets because i thought that the PvP system will be around forever. Then TBC came and all pvp was forced into Arenas