Ross is more or less right. Spider-man is street level because his focus is New York City. He usually stays local (although you could make the argument that he has done some adventures outside of Earth--Secret Wars and a short stint as Captain Universe--which could possibly elevate him to a cosmic hero). He usually deals with any number of minor villains from his rogue's gallery, who are just looking for a quick score but nothing world shattering. In that respect, the Flash and Superman might fall into the same category. However, unlike Spider-man those two heroes, especially Superman quite often find themselves pitted against major threats to the planet, galaxy, or all of reality, usually requiring the entire JLA or DCU to stop.
Now with that being said, Marvel has increased the number of major events over the last ten years and Spidey, more often than not, is swept up into the battle pulling him out of NYC and his civilian life. In fact, his recent full-fledged membership in the Avengers also takes away some of his street level cred as he finds himself in the big leagues. But the same applies to Batman, and while Batman may go on missions with the JLA, he remains heavily focused on Gotham City when fighting crime on his own. Spider-man stays within the confines of NYC when not with the Avengers. Superman often finds himself outside of Metropolis or even Earth when left to his own whims of thwarting any number of villains. Therefore, Superman is not street level.
Lastly, I'd like to say that I do believe powers do play a role, albeit to a lesser extent, in whether or not a hero is street level. Those with high-end powers tend to be global or cosmic level heroes as in the case of Superman, Thor, Silver Surfer, Wonder Woman, etc. Those with low-end powers or none at all typically have a localized impact on their ability to stop villains as in the case of the Punisher, Daredevil, Batman, Green Arrow, etc. But there are exceptions. Star-Lord, Adam Strange, Hawkman, and Rocket Raccoon are galaxy-trotting heroes who have no real powers or are on the relatively low-end of the power scale, but their exploits definitely differentiate them from street level heroes as they continually find themselves dealing with situations on other planets or dimensions.
