Of course travel to Mars, 150 million miles of hard vacuum and cosmic radiation away, is too dangerous and it will probably remain so for the foreseeable future. This question is phrased not, "should we go to Mars", or, "will we ever get to Mars", so it remains open to interpretation as to what currently constitutes "too dangerous". There is also a tendency to make assumptions that any current technological difficulties will be overcome in time somehow.
In May of 2001, Robert Zubrin, then president of Pioneer Astronautics and the Mars Society, told National Geographic Today, "We are much closer today to sending people to Mars than we were to sending people to the moon in 1961." If that were actually the case, then we would be landing on Mars sometime next summer. Enthusiasm and pragmatism are frequently at odds with each other and there are many pros and cons to the practicality of a Mars mission. But for the sake of brevity, I offer the top 5 reasons, in order of increasing importance, why going to Mars is still too dangerous at present.
5) If you think your cell phone service has dead-spots now, try Mars.
Unless we learn to exceed the speed of light in the next few years, communications are going to be a huge hurdle. Depending on the relative locations of the two planets, communication can take anywhere from 10 to 20 minutes; and that's one way. Throw in the fact that Earth would have to be above the Martian horizon to receive messages (and vice versa), windows of opportunity will further restrict the amount of information that's able to be exchanged during any particular interval of time.
Advances in laser-communications and increased bandwidth can alleviate some of the concerns by providing increased data capacity over radio waves. But even lasers have drawbacks, such as cloud cover, dust storms and the need to accurate pointing over vast distances. A series of relay satellites or even the orbiter itself might be able to intercept a signal no matter what relative positions Mars and Earth are in, but they still have to be on the same side of the planet as the Lander. And that will do nothing to shorten the lag time. While not necessarily a fatal flaw if all systems are go, in an emergency, such a delay could indeed be fatal.
4) In space no one can hear you scream, and they won't hear you scream in the Lander, either, without oxygen. The logistical problems of taking sufficient air, water and food for a round trip voyage is mind-boggling and at this point in time, insurmountable. When one notes the number of times per year the International Space Station must be re-supplied using heavy-lift Russian rockets, it quickly becomes apparent that the Mars ship will either be gigantic, or else there is going to have to be another way of providing food/water/oxygen for the crew's journey.
Other possibilities exist, such as pre-launching and staging supply stations all along the trajectory to Mars. That's pretty tough since orbital dynamics around the sun would affect the supply line, but it is still possible to place these supply depots into predictable, exploitable orbits provided something doesn't interfere with the mission itself and its timing in any way. One might even fly "robot supply ships" around and rendezvous with the Mars ship in route. Either way, it's going to take a lot of coordination, a lot of manpower and a lot of supplies. Until the ISS gets more self-sufficient for longer intervals than it is now, no manned Mars mission is feasible using currently available technology.
3) The dominance of outer space is going to require some serious "divide and conquer" tactics. Before we can go to Mars, we have to conquer the Moon first. Sure, we made a camping trip there a few decades ago, but in order to use the Moon as a platform for deeper space exploration, we'll have to place a permanent facility there. This means building, supplying and launching sophisticated space craft for building, supplying and using a Moon base for building, supplying and launching a Mars ship. Over-anxiously placing the cart before the horse in a mad dash to the red planet is a sure recipe for disaster.
The learning curve for the Moon base itself will not be short, and there would be lessons to be learned from a prolonged presence on the Moon that might apply to a Mars mission. Thus a prolonged training program on the Moon would be merited. That's a bit of a "delay" but certainly not a show-stopper, however, there is one underlying "danger" that will translate to every aspect of the journey to Mars: meteor impacts. One meteor impact can stop the show dead in its tracks.
Since the Moon has a gravitational field, there is a constant possibility of meteor impacts. Large or small, meteors will absolutely ruin a Mars mission, so even though the danger starts on the Moon, it doesn't end there. The prospect of having the Mars craft damaged by micro-meteor impacts at any stage of the journey raises the possibilities of catastrophic mission failure beyond what would be considered a reasonable risk. Without options for impact recovery or sufficient shielding for impact prevention, the uncertainty of the actual impact risk will delay any mission to Mars until that risk becomes a known quantity.
2) In space, doctors do not make house calls. There have been remarkable advances in the use of remote, robotic-assisted medical procedures. The time-lag between instructions leaving Earth and reaching the Mars module will make that quite challenging, but it is a surmountable problem with the use of properly trained personnel on the Mars module augmenting the process. But, there's one medical problem that can't really be dealt with in this manner: radiation. The amount of hard radiation that will bombard our astronauts during their many months long journey to and from Mars is definitely "dangerous" and without proper shielding, could be fatal.
These are unknown factors with regard to how much radiation to expect, how much the human body can take and over what duration. Additional shielding will make the module more massive, requiring more fuel and once again, we get into the area of sufficient resources. Closely related to this danger is another aspect of the sun itself and lack of adequate protection against hard radiation spewing out of coronal holes, and solar flares directionally targeting the craft and its fragile electronics.
Plus, the radiation danger may not end at the orbit of Mars, since the surface also receives more cosmic radiation than Earth. Here, you have an even less shielded Lander subjected to relatively less radiation, but still possibly at dangerous levels. A more shielded Lander is heavier which requires more thrust to attain orbit using more fuel and the resources dilemma is reinforced.
1) Then there is the biggest factor that makes a trip to Mars "too dangerous": the need for properly testing the mechanical integrity of the craft and all its peripherals under realistic conditions. Mechanical functions occasionally fail and the more complex the equipment, the more things that can go wrong with it. It will takes hundreds, maybe thousands of hours of testing of the actual equipment, subjected to the forces the craft will encounter, under identical environmental conditions in order to create contingency plans and reinforce the weaknesses. The best testing would take place in space itself where the cold/heat of space can be represented before you subject the entire structure to the stresses of deceleration after months in space. This will take time. Until that is done, Mars is too dangerous and a hasty mission is not only dangerous, but reckless.
There is risk involved in any exploration and one need only be reminded that Mount Everest was initially conquered with much less advanced equipment than exists today. But, people still die today. One big difference isn't the equipment, but the experience level of the people involved. Ingenuity and the human spirit has overcome much and will continue to do so in the future. Still, the first men to attempt Mount Everest failed. Two of those, George Mallory and Andrew Irvine, attempted an ascent on June 8, 1924 and died 29 years before Sir Hillary's successful ascent and descent. But, Mallory is still known to this day for explaining why he even wanted to climb Everest: "Because it's there." Mount Everest is STILL there. Mars is there, too, but we're in it for the return trip.