Wide beam vs Spot?
Spot for signalling, wide for general viewing. Spot is better for bad viz, penetration etc. Wide beam is nicer on a reef. Then there is issues if you will be doing photography/videography etc.. I have a little Intova that is great as a back-up for reading gauges, signalling and getting out of a wreck with. It wouldn't be so cool on a reef night dive though...
Replaceable vs Rechargeable Battery?
Depends on location and availability of resupply and/or charging points.
Handheld vs Canister?
Canister provides longer burn-time and more power (for the right torch head). It's bulky and hard to attach easily to a jacket-style BCD. Handheld can be more convenient, especially for the travelling (excess airline baggage) diver. Handheld can be 'regular' torch or pistol grip. Pistol grips are bulky and cumbersome. You can use a universal goodman handle, with a 'regular' torch, mount them on mask straps, stow them in a pocket etc.
Burn time?
Long duration dives require long burn time. Equally, you may be in a location where recharging/replenishing batteries is not convenient. Likewise, you may be doing multiple dives per day, without wanting the hassle of changing batteries regularly.
I have an Aquastar LED torch that'll do 18 hours with 3x C-Cells.
Brightness?
Brightness is relevant to the diving. Open water divers might want to illuminate a wide area, but not blind/scare the fishes. Cave/wreck divers might need to penetrate bad viz. There is such a thing as "too bright", depending on your needs. I don't like 'too much brightness' when wreck diving, because it can cause blinding backscatter/reflection on bad viz. ... 'just enough' is fine. I really like the OMS canisters for wreck penetration - value plus suitable performance. My Aquastar torch is small (back-up for tech, primary for rec) but nice and bright without blinding marine life.
Price?
For the average open-water diver, there's certainly no need to spend more than $100 on a torch. There's a bunch of decent ones in the $60-80 range. If you have specialist needs, then you're looking at (much) more expensive options.