I'd like to share my opinion as a non-marine player.
The battlebarge was made with 3 shields in the intention that it's 6+ armor would make their shields worth more against the weapons batteries in a fleet. GW felt as though this was about a 50/50 mixture of weapons batteries to lances, perhaps more in favor of the weapons batteries.
So if we think about it in these terms, compared to an emperor BB with 5+ armor on the front and 4 shields. We can note that if we have an equal number of weapons/lances firing (weapons batteries at 3 per lance) from a medium range and closing, then it would take firepower 27 weapons batteries to take down the shields on a Battlebarge, compared to 18 for the emperor. However since they are equally susceptible to lances, at 6 for the battlebarge, and 8 for the emperor.
Mathematically this means that the emperor is 33% better than the battle barge at taking lances to its shields, and the Battlebarge is 50% better at taking weapons batteries.
I don't think the problem with space marines is that they're low on shields or turrets. I think it's just a metagame thing, people love lances, especially newish players.
I think it's every ork player in the worlds nightmare to see a battlebarge with +1 shield and turret. It would mean that they are so much less likely to damage it through shooting (in order to get 27 firepower, they would need about 5 kill-kroozers in front arc) And boarding is almost out of the question, as a battle barge with bv of 16 when defending and a built in +2 from space marines, it makes the orks +1 for natural and likely +1 for outnumbering almost wasteful. Getting an advantage here is improbable.
Space marines are a fine fleet, sure they have some trouble with taking damage, especially on their strike cruisers, but that's what you get for basing a fleet out of cls. I honestly don't know what I would do to make them more competitive. Like I said... people are generally lance-happy, because it makes them have to think less, and worry about space marines, necrons, and even IN/Orks less. (space marines being the most common fleet I see, as people want to play the same fleet as their 40k army). Unfortunately Eldar players are rare, so the metagame isn't balanced in this respect, and the other fleets where weapons batteries are good against, lances don't get any worse against so that point is kinda meh.
Hopefully with the DE codex, and all the people that I'm seeing who want to buy into a DE fleet there will be some metagame change.