While not implicitly stated anywhere, I believe that they are either blocking important ports for these games and refuse to own up to it. (Or are too incompetent to figure it out for themselves).
-Or-
They are pulling the same shenanigans that they got caught red-handed doing and then got slapped by the FCC. ie: Throwing bad packets in the stream to purposely affect streaming video or file sharing quality. ( you'll notice that the first thing they do when you complain is suggest that you get their "upgrade" Blast service. That is, of course, the most stupid assessment of a problem I've ever heard.
However, over time, I have come to believe that it is the first of those two choices that is going on.
ie: Blocked ports - which affect certain types of online games, but not all of them.
Upgrading to Blast is not the fix. If you're going to find a Comcast product that doesnt block those ports, you'll have to go all the way to their business product. $$$$
An alternate choice is a business plan from something like MegaPath, where they run their OWN cable-style connection to your house. It's also expensive, but not quite as bad as Comcast. And, of course, you're not giving Comcast money, which is another plus. (about 120-150$ per mo.)
To the best of my knowledge, you will never get Comcast to unblock the necessary ports, so you'll either have to try a different ISP or change your game type to something else that works fine with Comcast.
They do not care about gamers, as you can see from these 2 or 3 pages, they never pitched in, reviewed or discussed these issues in this forum, they will do nothing to solve your problem.
Once again, the solution is at least one of these:
1. Change your game type
2. Change your ISP (not all ISP's block the same ports)
3. Change to a business plan (I believe, because businesses may require more ports open/unblocked)
Do not forget, this entire thread assumes that you are computer savvy enough to have removed all other possibilities such as performance issues, malware, viruses and connection problems and KNOW that the issue is not in your system, modem, router, cables or connection.
* I am a professional technician by trade and this is my (long-winded) opinion *