Well currently I don't really worry about which slot stuff comes from. I just take the stuff I like. I tend to aim for lots of scoring units so I avoid ICs and transports. At times I have resented having to take troops and HQs if there aren't any good enough.
5th is going to mean a really radical change in my army design, possibly down to which armies I actually play. The chaos army I had been planning, with just a couple of small units of zerkers for troops and a lot of small elite and heavy support units just wouldn't work in 5th. I would have 2 scoring units, and things like my units of 1 oblitorators would give away a lot of points in the total annihilation set up, where currently they are more or less disposable. Transports look much more viable now since they don't really reduce my scoring unit count very much, but can move my vital troops around the place.
So now the armies that look good are the ones that have troops which you actually want to take a lot of. You want to be able to take the minimum amount of stuff that doesn't count as troops, while still being able to move around and kill the enemy.
I'm not sure even if the same armies that are competitive currently will remain good in 5th. Falcon based eldar won't work in the current configuration at all and my Tau will suffer too. Marines should be able to cope, especially since they are able to achieve a lot with troops (and in various codexes have terminators, bikes, scouts, and assault marines as troops).
By contrast, some of the armies that are less competitive now are heavily based around troops. Sisters of Battle and IG, for example, may be quite good - especially since IG tanks are taking quite a buff. I think that a squad of sisters jumping out of a rhino and shooting their bolters and melta/flamer weapons will be really quite serious. Lasguns will still be lasguns though.