Needless to say, there be spoilers in here… If you have not completed Mass Effect 3 or haven't yet played the Citadel DLC than you have been warned and it's your own fault!
During the party in the Citadel DLC, provided you have Kaiden as your Virmire survivor and have invited Kaiden, and at least Liara and James Vega are invited, a discussion comes up between Liara and James discussing the merits between physical strength and biotics. During this discussion, Kaiden mentions that through training hard he has improved his biotics and is now able to Reave.
Miranda, Liara, Jacob and Jack will all comment on this revelation in surprise, almost in disbelief that a L2 biotic is able to Reave. Why is this such a surprise?
Best Answer
First, some speculative background:
Considering the only people in the ME games that we see can use reave are extremely strong Asari (Samara and Aria T'Loak) and Commander Shepard (possibly one of the most powerful human biotics ever), we can hypothesize that reave is an extremely 'high level' biotic ability that only a very few can pull off.
Taking this into mind, and coupling it with what we know of the L2 implants from the Mass Effect wiki:
I'd imagine that the majority of people with L2 implants fall into the 'hardly stronger than an L1'. This and their general mental instability (remember what happened in ME1) has lead to the public perception that L2s are not fit and possibly will go insane before being able to master the stronger biotic powers.
Also, Kaidan did mention in an earlier conversation with Shepard that he can 'spike higher than most L3s with the exception of Command Shepard'.
This does come at a significant cost of extreme discomfort, so I'd imagine they are more surprised at the fact that he can tolerate those levels of discomfort to pull off such a difficult ability without going crazy.