You have weak anti-armor. The lascannon has a nice 66% chance of hitting, but it's only one gun. You're probably not going to see too much armor/high T units (ones with a high enough T to need an S10 weapon to deal with) in a 500 point match, but it's still a very real possibility (most people field at least a token piece of armor, simply because it requires a special counter to deal with).
The commissar's going to die if used on his own in frontline combat, because he's a high priority target. He might be tough as nails, but weight of fire means that statistically he'll be hit, receive wounds, and fail enough saves to be taken down. His strengths are mainly in shoring up other infantry units.
I'm also generally wary of putting too many options in a single unit, though I'm not sure how well founded this is.
This is my 1000 point infantry spam army. It slaughtered a space marine army, despite getting a venerable dreadnought deepstriked into its heart.
IG: 1000
Company Command Squad +50: 65
-Mortar +5
-Grenade Launcher +5x2
Platoon Command Squad +30: 35
-Vox-caster +5
Infantry Squad +50x5: 365
-Vox-caster +5
-Commissar +35
-Grenade Launcher +5x5
-Autocannon +10x5
Heavy Weapons Squad +60x5: 435
-Heavy Bolter +5x9
-Lascannon +15x6
Veteran Squad +70:
-Heavy Flamer +20
-Flamer +5x2
It revolves around the idea that more dakka is best dakka. The central infantry squad (formed by combining the five squads in the platoon with the Combined Squad rule) can fire 68 lasgun shots at 24" (102 within 12") using the 'First rank, fire! Second rank, fire!' order, not counting the five autocannons (10 attacks at 48", good against light armor) and grenade launchers (crowd control and light anti-armor at 24") it has. If properly distributed, the heavy weapon squads turn the entire map into a killing field, for armor and infantry, and the infantry can slaughter even a concentrated charge with their help. The veterans are just there because I needed two troop choices (the platoon counting as one troop choice), but their flamers deal with anything that gets closer to the infantry/can be used to assault a position if need be. In theory, at least. I haven't played enough games with it to know for certain. It did leave a space marine player hiding behind some buildings on the other side of the map, unable to advance, despite dropping a dreadnought on me, though.
Of course, take my opinions with a grain of salt. I have fairly limited experience, though I learned a good deal from Neruz, before he was cruelly slain for being flamed by the foul villain Pathos. D: