it doesn't quite fit
Back in the original natural-selection, each team had their own distinct traits. The marines were slow and had cumbersome technology, but came with great force, while the aliens were agile and elusive (for the most part) and attacked with surprise. The roles were cliche perhaps, but each team was completely different and it was a great dynamic.
For the sake of this post, I will focus on the specific attributes of the marine team. They were slow.. really slow. A good 50% slower than the aliens, and that's if the aliens weren't bhopping. They were also predictable. Always starting in the same location, while aliens could be in any of three possible places. When a marine was parasited, it was a big deal, because he knew that every alien on the map now had permanent knowledge of his location, no matter where they were. Even without parasite, it was easy to monitor marine actions because they were always in plain sight.
Because of all these factors, marines were essentially forced
to move in groups. Their slow speed meant that dying was a major set back, taking ages to regain what ever progress had been made. Their predictable movements meant that encounters were inevitable, they couldn't merely slip under the radar, they had to use firepower to overwhelm whatever obstacles they encountered.
This also meant that an unseen marine was a deadly marine. Failing to keep track of an enemy marine could be devastating to the alien team, and was frequently the cause for lost hives and games. But it wasn't easy, a single footstep would give you away. It took time skill and patience to get to the ideal location to attack from, and it was rewarding.
In NS2 however, this seems to have changed greatly. Aliens are now only marginally faster than marines, and no more elusive. Marines can appear anywhere at any time, and most alien structures are so weak that they are essentially undefendable once a marine gets up to them. The entire marine team can prepare a shotgun rush on a hive location, and the aliens will only know once the marines are directly outside the room. There is almost no reason to move as a group, because no matter where you die on the map, you know you can get back to that exact location in no more than 30 seconds tops.
In my opinion, the marine team's extreme mobility is destroying the strategic dynamic of this game. Its gotten to the point where it is the marines that are the sneaky ones, appearing where you least expect them, attacking quickly and devastatingly. Its the aliens who are slow and predictable now.tl;dr remove sprint from the game