To me, Hypnotic Pattern seems quite strong for a 3rd Level Spell. The main reason I think so is that it can affect a huge number of targets, and a creature that fails the saving throw will not get another chance and may very well be taken out of the picture for almost the entire combat encounter.
I’m aware of two of the spell’s relative weaknesses. First, a creature can only be affected once when the spell is cast (other than with the 2nd Level Spell Web), even though the spell still requires concentration. Second, the apparent effectiveness against large groups of creatures is somewhat reduced by the fact that some creatures will usually succeed on the saving throw and will wake up others. On the other hand, this requires their action, so one might in fact argue that this makes the spell even better for the first round of combat.
All these things considered, I’m wondering if it would be balanced (possibly even more balanced) if Hypnotic Pattern were moved to Level 4. Or am I missing something?