I don't particularly have MP issues as an AST if things go smoothly, however when things get hairy I do find myself struggling to keep up with the healing output.

1) Luminiferous Aether is exactly like Shroud. That set MP refresh that doesn't really scale with your ilevel (unless they've changed it recently and I missed something). It's gonna refresh the same at i160 as it will at i190. This isn't so bad in the beginning, but as soon as your MP pool starts expanding, it becomes really noticeable. It's basically the skill WHMs have been complaining about since 2.0, except AST didn't come with an Assize in 3.0 (I suppose extending LA with Celestial Opposition is kind of supposed to help? But you'll have to spend 30 seconds not using LA while waiting for CO to come off CD so that isn't really super reliable...).

2) You have one instant heal that costs you no MP. Essential Dignity is like Lustrate and Benediction's temperamental love child. If you use this on someone with low HP, you basically Benediction'd them and that is worth putting it on CD. However, if you were a second too slow on that button and the other healer happened to heal that tank/DPS/whoever. Your heal's potency just drastically dropped and you lost your only instant heal and one that costs you no MP at that.

I guess you could almost say that AST is kind of like a 2.0 WHM trying to do 3.0 content? XD