Is this waterfall characteristic of QAM or OFDM (or something other) modulation? I recorded this signal at 2.45GHz, it is coming from a camera.

Is this QAM or OFDM ? 

Based on those side-lobes I would guess OFDM but I am not sure.

Also there seems to be an interesting preamble at the top.
The sharp rectangular shape of the spectrum, with the immensely flat top definitely says "OFDM, with suitable whitening/PAPR reduction"; what constellation is used on the individual carriers is a bit hard to tell.

Can you, with a no-samples-left-out spectrogram (e.g. gr-fosphor sink, or by playing back your recorded file slower and using a suitably high update rate and suitable low number of FFT points in your QT waterfall sink, determine how long the OFDM "burst" is?

Take a bunch of samples from the end, and a bunch of samples from the start (like, one fourth of the burst duration), and calculate a cross-correlation. The maximum correlation should give you the time when what is used as the cyclic prefix at the end of the burst starts; that way, you'll be able to make an educated guess at the symbol number that makes up the CP, and by that the OFDM symbol length/IFFT size. Take frame, and try to manually resample it, so that both CP starts are exactly that (guessed) IFFT size apart. FFT the hell out of the frame; because your resampling had the purpose of time-stretching your signal so that the length of it actually (hopefully) matches the transmitter's sampling, you'd have a timing-synced demodulator. 

Look at the spectrum and the constellation plot of the resulting signal; maybe you can already guess the constellation used to modulate the OFDM subcarriers from that.