Ok, so this post is on tactics for different styles of armies here we go:
GUNLINE!: Gunlines are very hard to play in 5th but they are still possible with a little practice, first you need things like devs, prefferably things that can infiltrate, you need to set these up in fire arcs and in cover, keep these with long range weapons. Your troops need to get ready for a assault and get into cover and make small, unavoidable fire zones if possible.
Elites should be there to blast up the enemy then hold them up when they assault so the troops can overwhelm them.
Fast attack units need to be hidden but ready to attack on the first call. If you play marines then try a land speeder or 2, drop these in behing the enemy and stay out the way and blast their fast assault troops (many fasy vercles have atleast 36'' range guns). Generally you are going to have to kill the enemies fast stuff quickly. In a gunline army prioritising targets in essential.
Competive score: 5 (out of 10) these armies are not to competitive at the moment but can be fun and some players can make these armies work.
FAST BOYS: These are armies like saim-hann (Fritz your really good at this style) or biker and land speeder Dark Angel armies. For these armies to work then they need to stick to cover, generally they can be killed quite quickly if exposed so they need to take out the enemies biggest threats before making objective grabs on turn 5. These armies need more than 1 wave otherwise if the game doesn't end on turn 5 then your dead but if you have a second wave then you try again on turn 6/7.
Deepstriking and target denial is good idea for these armies. If your opponent doesn't know when your going to appear or where then you gain an advantage plus you have less turns to get killed in before your objective grabbing sessions. Target denial also helps keep you alive. If the mission in anilation then these armies need to concertrate their fire power then get away before retaliation.
Competitive score: 4-9 (out of 10) These armies need a good general that knows his army and is awsome in the right hands (like Fritz's hands).
BALLANCED: Generally you find armies that have a mix of fast units (used like in the GUNLINE or in the FASTBOYS), power units (which get into position and blast the minds out of stuff, slow units and deepstrikers. All of these units are used differently in each codex so I can't help specifically here.
Competitive score: VARIABLE: these armies vary from codex to codes.
DEEPSTRIKERS: The boys that can come in wherever but are relatively slow (the fast deepstrikers are in FAST BOYS) these are things like termies and I know this is not deepstriking but things with infiltrate or scout. With these units you need to destroy your opponents biggest threats then crowd on about 1/3 of objectives and contest another 1/3 to guarantee a win, if you have to kill your opponent then kill things that give you easy KPs.
Competitive score: 4-7.5(out of 10) These armies are hard to play but when you get the hang of them they can be fun/effective.
TRANSPORT/MECHANISED: These armies are fully mech and have to rush troops forward into good positions, the hard hitters of these armies need to attack the enemies big threats and troop choices in objectives, other places have covered the uses or rhino/razorback walls so I won't, but they are awsome here, with these armies you need to get to places quick then HOLD THE LINE! Biker armies can be used like this too but need some more thinking.
Competitive score: 5-8.5(out of 10) These armies are simmaler to FAST BOYS but are more forgiving and slightly less effective.
Sorry if I have missed army types or have got things wrong, correct me if I have made a mistake CC is welcome. Some of the competive score are dependant on experience and style of play.