I see a devastating lack of powerfists on your sargeants.
And scouts aren't really worth it.
Also, while it may be a decent idea in July come 5th edition, your current loadout with flamers and missile launchers and frags doesn't really work out well.
The missile launcher needs to stay immobile to fire.
The flamer wants to get close.
Frags are for actively charging enemies in cover.
In the future you might be able to break each squad into two, one with the missile launcher that stays put and the other five with the flamer and perhaps a powerfist that moves aggressively forward.
I also know you don't have a space marine codex yet, but you may want to look into the traits system where you could have two plasma guns or two flamers instead of 1 heavy and 1 special weapon.
Traditionally most competitive marine armies min/max, which is to say they take as many 6 man lascannon/plasma gun squads as possible with a few tanks and such. See Steve for his current ultramarines list, however in future editions GW seems to be disallowing such squads.
