About 4 out of 10 GMs I know openly claim to prefer "street level games."
Said GMs, however, usually have a hard time putting up anything interesting and most of the time end up reverting to type and leaning on the crutch of (mini) "supervillain of the week" (thus chucking out the whole "street" element from the game). This is probably not their fault, since most GMs have the trad paradigm too deeply ingrained in their brains so the moment they are faced with "lego box systems" like M&M or minimalist ones where everything is DIY their heads only draw a blank because they're conditioned that "if it isn't written in the book, it doesn't exist."
While 3E discarded the PL 6 as lowest tier for games, even at PL 6, single street thugs were never meant to be a "life threatening battle" for a superhero (it's M&M, not Mystery Men). This, however, does not mean you can't do "Batgirl of Burnside" with M&M, but you need to know your tools in order to turn bottom feeders into a fun opposition (remember, FUN is the operative word, not deadly, not overpowering, fun).
Kit One: Biker Gangs and Vehicle Combat
NOTE: While the core book has no proper "vehicle combat" rules, it's not hard to extrapolate the basic rules for what happens when "man vs. car" happens. For regular "vehicle vs. vehicle" conflicts, however, just use the vehicle's base stats as usual.
Car vs. Character
(all extrapolations courtesy of Relative Speed principles)
* Characters inside a car or bike add the vehicle's speed class to their melee attack and damage ratings while the vehicle is moving.
* Characters inside a car or bike subtract the vehicle's speed class from their ranged attack's rating.
* Characters attacking from inside a car or bike are assumed to possess the Move By Action advantage (for obvious reasons, attribute it to Descriptors).
* When aiming specifically at a vehicle's driver, characters inside a car or bike add the vehicle's speed class to their dodge and parry stats while the vehicle is moving, and they add the vehicle's toughness rating to their own.
* Cops firing from behind the open doors of their parked car enjoy full cover.
* Characters attempting a Trip attack on a motorbike use the toughness of the object they're trying to jam in the wheel versus the bike's strength
Basic Statblock: Biker Gang Banger (basic NPC: thug)
Numbers divided by a slash indicate (vehicle/driver)
* STR 1/2, STA 2, AGL 1, DEX 1, FGT 2, INT 0, AWE 0, PRE -1
* Equipment: Leather jacket (+1 Toughness), light pistol, cell phone.
* Skills: Athletics 4 (+6), Vehicles 4 (+5).
* Offense: Init +1, Melee +8 (Damage 10), Pistol -9 (Ranged Damage 3).
* Defense: Dodge 10/8, Parry 10/8, Fort 8/4, Tou 8/11, Will 0.
Kit Two: Police Crackdowns
Remember that the Aid and Team Attack actions are in the book for a reason. Send in a big enough swarm of minions (10 per PC minimum) and, statistically, you can just waive all the extra dice rolling. Your swarm will only roll one attack check per turn, but it will be one with a +5 bonus to attack. Furthermore, if your swarm is a minimum of 20, your lead attacker can enjoy both a +5 to attack and a +5 to damage.
Copyright: DC Comics and Paramount)
The Catch: The moment your swarm's numbers no longer sustain critical mass, the bonuses are gone.
While usually only the police and the army are organized enough to pull these stunts off, particularly organized crime (like your token tacky martial arts crime syndicates and low-tier, non-superpowered ninja clans) will also use these.
Kit Three: Blockades, Kill Zones, and Dragnets
While (again) gangs are usually not organized, drug cartels and terrorists are prone to rising blockades full of armed people ready to fill would-be tresspassers full of holes:
* All members of the defacto firing squad are assumed to fire from cover (full).
* Every turn characters are within firing range, they're target of two attacks: A regular firearm attack, and a Demoralize attack with a +5 bonus (unless the target is bulletproof and has no reason to fear standing on a kill zone, that is). Since we're talking "street level," it's fair to assume most characters can't fly, and thus would rely on mundane skills to get past a blockade, that's where the Demoralize action becomes crucial.
(add extra vehicles and armed goons to taste)
Basic Stats: Blockade (manned by at least 20 thugs)
* Skills: Intimidation 9
* Offense: Init +1, Pistol 1 (Ranged Damage 3).
* Defense: Dodge 7 Fort 9/4, Tou 9/3, Will 0.
* The nature and difficulties of the Athlerics/Acrobatics checks to get past a blockade will depend on its composition. Just remember that characters are Vulnerable while climbing, so trying a frontal approach to zip past obstacles they can't surmount on a vertical jump will turn into a Re-enactment of Kagemusha's ending.
When talking about a kill zone trap, usually the heroes are lead somewhere with a single access point, same that it's getting blocked as soon as they're at the center (example: two buses blocking an alley exits). While characters with the right skills may realize before the trap closes, failing to do so guarantees an unpleasant time.
A Dragnet, finally, is a Crackdown where also the police happened to establish a perimeter around a building, so players trying to escape the Crackdown also have to go through a Blockade. While characters escaping through the rooftops have a way past the blockade secured, the police will usually have at least one helicopter ready as well... and if SWAT is involved, this means elements are probable also waiting in neighbouring rooftops, turning each into a separate Crackdown.
Kit Four: Snipers
Modern military rifles have an effective range of at least one mile (and that without resorting to old sniper tricks like slightly parabolic shots), and rifle scopes exist for a reason. Barring telescopic vision powers or wearing Batman's cowl, characters have a -5 on Perception checks to detect a sniper's presence stationed in a proper Hawk's Nest (so chances are the first shot will always catch the target Vulnerable). After the first shot, however, any character with the Investigation skill can point at the sniper's location with a check.
Kit Five: Mastermind Showdown
Sooner or later, the heroes will have a showdown with the Crime Lord in turn, who will be either waiting for them at the penthouse of his ominous skyscraper, or making his way to the heliport in the rooftop to make his escape. Either way, once again we resort to the Team Attack action. Give the Crime Lord NPC at least 10 thug minions as buffer so the Crime Lord has a constant +5 bonus to Dodge (and the human wall would keep him away from melee range).
Alternatively, if he is making his way to the helicopter, make it a "timed challenge": Give the Crime Lord 10 minions times the number of rounds you're giving he heroes before the Crime Lord makes his escape.
I may add more as they come to my mind.