I miss the days of energy denial mesmers... back when you could Energy Drain and force the other guy to lose 20+ energy, Energy Tap to cause a loss of 12 energy, and 12 energy off Energy Burn... yeah, that's right, 44 energy in the course of a few seconds. My old guild had a nice little spike build in which I'd energy-spike a monk, and everyone else flies in to deal as much damage as they could before there was enough energy to heal.
Infinite minions were also good fun. Maintaining 200+ Bone Minions FTW, indeed.
I do NOT miss the refund points, though. Ugh. "Whoops, hang on guys, I can't GvG yet, I need to go off and farm a bit for more XP so I can use my PvP build." And how about those skill captures? You had to anticipate some of the skills coming out in order to get it before they used another right afterwards, but not too early or else you walk away with another common skill you already had!
The Proph-only days were good ones overall, though. Strategy hadn't really developed beyond Spirit Spam or IWAY in PvP, and mesmers were thought to be generally useless because they couldn't DPS like elementalists. While that made it a huge pain to find PUGs, I liked being my guild's secret weapon.