The problems with the codex can be divided into two areas. The first are problems with the datasheets. These are issues with individual units that make them less than usable. The second area is problems with our detachments and formations, which just aren't up to the levels of those found in Space Marines or Dark Angels.
Datasheets
The most straightforward problem is the lack of parity with Codex: Space Marines on certain units. Terminators need to go down in points cost, Vanguard Veterans need discounted weapons, and Scouts need WS and BS 4. Our Dreadnoughts and tanks need to be able to be taken in squadrons and get the appropriate bonus for taking a full squadron. Devastators should have access to grav-cannons.
Assault Marines need a reason to be taken. Giving them access to Eviscerators would certainly help make them a real threat. Personally, I'd like to see them be able to take 2 special close combat weapons rather than special weapons, preferably with the same discount that Vanguard Veterans get. That way, they've got enough access to power weapons to be a decent assault unit, but they still aren't competing with the more elite options because they only have 1 base attack and can't take quite as many power weapons.
Blood Angels need some anti-air ability. The easiest way to add this would be to give them access to Hunters, Stalkers, and Stormtalons. A more interesting way to add it would be to give Baal Predators the option to Skyfire. That alone would turn this seldom taken tank into an interesting unit. Sure, it's not as good at taking out flyers as a Stalker, but it would be more flexible since it can shoot at ground targets as well. As a bonus for taking a squadron of 3, give Baal Predators Ignores Cover. That fits well with their armament and would complement both of their roles.
Give Blood Angels access to the Land Speeder Storm or something equivalent. It makes no sense for them not to have an assault transport for their Scouts when Codex: Space Marines get one.
There are, of course, other issues such as giving Tycho some sort of close-combat weapon, but those seem minor and kind of nitpicky to my eyes. I think the above would straighten out most of the problems with the units themselves. Honestly, the datasheets are the lesser problem here. It's Formations where Blood Angels really fall short.
Detachments and Formations
Obviously, Blood Angels need a composite Detachment made up of Formations, similar to the Gladius or Lion's Blade Strike Force. Just like those, it would be based around a Demi-Company. In the Blood Angel's case, the Demi-Company would have Red Thirst and Objective Secured. If you take 2 Demi-Companies, then all of their dedicated transports are free. These rules are all pretty basic parallels to the other codices.
The real question is what the bonus would be for taking the composite Detachment, which I can only assume would be named Blood *something* Strike Force. For the other codices, this benefit is an extension of the Demi-Company bonus. A Space Marine Demi-Company has one Combat Doctrine while the Gladius Strike Force has three. A Dark Angels Demi-Company fires overwatch at BS3 while the Lion's Blade Strike Force fires at full BS.
The benefit for the Blood Angels is less clear. The obvious thing to do would be to have them get +3 Initiative on the charge, but I don't think that would be very good. The benefit should definitely help with assault. An extra point of Strength on the the charge would actually be pretty good. However, my choice would be Fleet. It helps us get into assault reliably, which is something that we dearly need. It also matches our reputation as one of the fastest Chapters and ties well into our Fast vehicles.
Beyond the Demi-Company, we need some Auxiliary Formations. Some variation of the Armored Task Force, 1st Company (probably called the Archangels) Task Force, and 10th Company Task Force are all obvious inclusions. I'm sure we'd also have a Librarian Conclave that would have to include Mephiston, which would be incredible.
Beyond that, I'd like to see a Formation of 3-5 squads of Sanguinary Guard, with the option to be led by Dante. The benefit would be something about matchless weapons, which would allow Encarmine Blades to re-roll all of their missed to hit rolls, rather than just being Master-crafted. That would be good on the table and speed the game up so much that I would take it just for that.
There would also have to be a Death Company Formation, made up of 3-5 squads of Death Company or Death Company Dreadnoughts, with the option for one or more Chaplains. I'm less sure what the benefit would be here, but based on the Strike Force Mortalis Formation, getting an extra attack for being outnumbered in assault would work.
Those are the obvious ones I can think of, but I would like to see Games Workshop come up with some good combined arms Formations like they did for the Raven Guard. The Angel's Wrath Intervention Force, Archangel's Sanguine Wing, and Angel's Fury Spearhead Force are all promising starting points, though they'd need to have their restrictions loosened so that they could fit into an army alongside a Demi-Company.
Those are the changes I would make if I had the chance to write the Codex. What do you think? Are they over the top or not enough? What would you like to see?
You can also weigh in at the Ex Libris Mortis Forums.