You consistently get more for less than any of the older codices. Thus why when ever a new codex comes out, all anyone sees/says on the Internet is "Why aren't they updating my army! IT SUCKS RIGHT NOW!" Well, that is because, to put it simply, all old armies are essentially bad.
Its not that you can't win with them. Its not that they weren't good. Its just that they don't have the potential to build the competitive lists that the newer 5th edition codices.
In time all the older ones will be updated. Even Dark Eldar and Necrons. Yes, it does suck in the meantime to be playing the older rules against the bullies of 5th edition (Imperial Guard, cough cough).
What to do in the meantime: Play for fun. Instead of taking what units you can to try an be competitive, take what you like. Come to the table going "I am more than likely going to lose, but I will have fun if my *insert unit here* utterly destroys that psyker battle squad."
I know Jawaballs wins all the times with Blood Angels, but he does it without Jump Troops. To me, this isn't as much fun. I want my Jump Packs. Therefore, I play with my Jump Packs. I have a rough time right now in one on one games, but I give it my all and take the small victories to heart. That, and know in my mind that my day in the sun will come and I will eventually have the newest 5th edition codex.
Another thing to do, is play 2 on 2 games. It is a very good way to play to your army's strengths and have a lot of fun using what you like, while your team mate covers your deficiencies by playing to his strengths. When I do 2 on 2 I like my allies to have shooting and/or numbers. This way, taking Jump Troops is beneficial to him, and his numbers and/or range cover up for how small and assault based my army is.
So basically, have fun against the 5th edition power house. If you can't beat it, just fight it your hardest and enjoy what you were able to achieve.
To the right is a poll on what you think is the most powerful 5th edition codex.
Results
Which of the 5th Edition codices do you think is the most powerful?
Chaos Space Marines 1 (1%)
Chaos Daemons 4 (5%)
Space Marines 6 (8%)
Space Wolves 9 (12%)
Orks 2 (2%)
Tyranids 7 (9%)
Imperial Gaurd 43 (58%)
Eldar 1 (1%)