Researchers would first have to send probes to the planet to determine the likelihood of ELE (Extinction Level Event).
You can ship 500 people to a planet, but if they don't predict a massive electrical storm coming along, or fire rain or whatever, the entire population could get wiped.
Others have mentioned food, water, etc.. pathogens would be a major consideration, too.
The colonists would need to either orbit the planet for a long time to sample the air for potential pathogens and run experiments, or to establish a research base on the ground that isolated them from the environment until they could make sure no existing pathogens exist that would wipe the entire colony.
This is the "why are you taking off your helmet even though the atmosphere is breathable, you f'ing idiot!?" factor.
Chances are a bit slim for this to happen though, contrary to popular belief. A planet already has an establish ecosystem full of predators, prey, specialized creatures fitting in niche areas.
Humans showing up wouldn't likely match any pathogen or bacteria or what-not that currently exists....but, since bacteria and viruses reproduce so rapidly, they could suddenly jump species and start wiping out humans. EG: there could be a creature or plant that has a pathogen in it that has reached an equilibrium in the enviromnet. Humans show up as an invasive species, and provide a new host to take advantage of. The humans might have an immune system that can adapt to it quickly, or it could be somethign that wipes them out quickly.
Generally speaking, the bigger concern would be the humans act like an invasive species that messes up the environment a lot if not careful. They might have stow-away pathogens on their ship, or bugs or what-not, and those things might show up and start wiping out the pre-existing biodiversity.
But, getting back on track....
1) is there anything on the planet that can immediately kill us? Be it massive predators, weather events (ion storms, fire rain, etc), pathogens, etc? If it's an Earth-like planet, then chances of radical weather is probably slim, but even Earth has crazy hurricanes, tsunamies and such.. so, stilll
.. from there you go down the survival list...
3 minutes without air
3 days without water
3 weeks without food
2) is the atmosphere enough to support humans (assuming no pathogens to kill us). If not, do we have the resources on-hand, or available on the planet to make oxygen scrubbers or such until humans can adapt?
3) is there a water source to fill the needs of our colonists (b/c humans consume a lot of water, not just drinking, but for hygeine, cleaning things, etc).
4) is there an ample food source? Are crops and animals we're used to able to sustain there, or are there food sources we can use naturally occuring there?
1) if we're using a technological crutch to sustain ourselves, will we get wiped out if we lose that crutch (extinction level event)? EG: Matt Damon on Mars... if he lost his shelter for oxygen, his food, his water.. he's screwed. If humans require some advanced technology to survive, then some ion storm shows up and blasts it all.. oops
2) do we have the mind-share to sustain ourselves? IE: do we have enough people learning to be farmers, ranchers, builders, etc. to maintain the living conditions? If you're using advanced technology to sustain yourself, and you can't educate the next generation fast enough in genetic modification of foods, gene therapy of humans for medicine, etc, then you're rate of survival starts to decline simply because your means of education is drying up.
3) Are we acting like an invasive species? Humans tend to grow into the area they're given, and terraform it along the way. As humans start to cut down trees to make fields, add industry, etc, they'll just end up with the same situation we have on our planet; where we lower the biodiversity of the natural planet by destroying ecocystems in order to sustain ourselves. Eventually, humans have to act like caretakers of the world, or come to some kind of balance with the ecosystem to ensure it can still sustain them.
4) do they have sufficient waste management? A colony won't do so well if it's not disposing of it's own waste efficiently.. which cuold become a cesspool breeding source for pathogens. They might want to use their own feces to create energy (methane) or fertilizer, but if it's mihandled it could contaminate everything and give everyone a really bad time.
a) humans will adapt over time. Humans are the millions-year culmination of DNA / Biology adapting over time. So, given enough generations, humans might adapt to better handle the environment.
b) with sufficient technology, the colonists could hyper-evolve, essentially rewriting their genetics quicker to better cope with the new world (think of the movie Pandorum, where the colonists were injected with an accelerator to help their genetics evolve quicker to adapt to the planet during their own generation instead of waiting for many generations to do so... which went to utter crap when the ship screwed up and the colonists mutated into cannibal monsters).
c) are they producing enough replaement population to grow and make-up for unforseen attrition? EG: if there's a plague that comes out of no where and wipes the colony, is there enough population to bounce back? Are they breeding enough?
With enough technology, you can "Sci Fi" deus ex machina your way around a lot of problems.
1) the colonists may have accelerated genetics, so they can quickly adapt to the planet, including all pathogens and such
2) they might be genetically modified to not have to "eat" normally or use the bathroom. IE: they are genetically optimized to take in nutrients, maximize usage, and recycle all byproducts into somethign useful. This would require a very high-tech society. It elminates a lot of burden of
2) the colonists have the ability to grow food, even protein, wihch takes the burden off feeding themselves
3) the colonists have "easily replaceable tech".. so if an energy storm wipes out stuff, they either can easily replicate the stuff they need or they have advanced brains and can quickly create advanced tech out of sticks and stones. (Rick & Morty that crap together).
etc, etc, etc
So, food, water, air... but also check for pathogens, statistically analyze planet to make sure not over-populating it, etc.