Having less guns allowed each gun to be completely unique. In Mass Effect 1, there was only really 3 guns.. Whatever Gun had the best damage/Heat ratio, Spectre 7, and then Spectre 10.
Everything else was just chaff to be sold.
In Mass Effect 2, every gun is completely different to the other guns.
The Starting pistol for instance, is more like a standard 9mm gun in a shooter game.. lots of ammo, little damage per shot, but consistent. The Carnifex Hand Cannon on the other hand, is like a magnum. Fewer shots, massive damage per shot, meant for handsniping, rather then suppression.
The SMGs come in several flavors - There's the 3 shot model, the 5 shot model, both of which are entirely different beasts. The 5 shot model is better for suppression (firing a shit ton of bullets to force an enemy into cover), whereas the 3 shot is better for tactical hits.
The shotguns come in the 1 shot, 3 shot and 5 shot cartridge models, and all of them are also very different. The 5 shot is for the people who like to fire off a lot of rounds, and have access to a lot of ammo. The 3 shot version is the in between, and the 1 shot aka "biggest burst damage in the game" is only for people who are, to quote Denzel Washington "surgical with this ************".
For sniper rifles, where's the Viper, which is for noobie snipers, or snipers that want the chance to correct their aim after a shot. The Widow/Starting Sniper rifle are standard sniper rifles, 1 shot per zoom, deadly headshots, etc.
The complaint about the lack of guns is unfounded, because all of the guns in Mass Effect 1 were horrible. Every single one of them, apart from the Spectre series. The massive amount of choice was an illusion in ME1 because every gun sucked.