so imo::
in summoners rift (SR) a good team is comprised of different rolls:
tank, ranged dps, caster, support, and tanky dps
maybe changing a couple like support for another caster or tank for another tanky dps, but basically filling these primary roles is the basis for a good team. other team synergies come into play, but this is the basic outline.
in dominion, a basic role system is needed as well, but the rolls are different based not on the original intended roll (like shen is a "tank" and annie is a "caster") but how well they attack or defend towers. i think that the best team is comprised of 2 defenders, 2 attackers, and 1 distracter. these rolls are also relatively interchangeable in the sense of akali is damn good at attacking, defending, and distracting. making her completely top tier in dom, you could swap her out for any role and still have a beast team. multiple champs can fill multiple roles, but, like in SR, the core roles must be filled to have a good team. other top tiers will be really good at multiple roles or godly at a certain role. heimer for example, is horrible at distracting but amazing at defense, and in late game can be a really good attacker as well. imo AP shaco is the best defender in the game, as well as a good distractor, but a bad attacker.
more core ideas for DOM are that movement speed wins games. in theory if no team ever engaged, the team that moved faster would win. map control is very important too.
what i think is bull**** is the point system they have implemented. singed is decent at attacking and defending but he is an amazing distractor due to his movement speed, disruption, and survivability (core stats for a distractor) but he will usually have the lowest points on his team if played right by distracting the other team into being out of position so the rest of your team can take and defend turrets. he wont cap or get kills, but he could very well be the most important member of his team.
in a shorter version, if you had 5 good attackers, you could never hold towers and you would lose. if you had 5 good defenders you could never take towers and lose. if you had all distactors the game would be incredibly random and you would still not be able to take towers back. a good mix of all is required. i suppose a distractor isnt required but if you notice the other team having a rammus pulling you out of position caping random points all the time you will probably lose.
simplest form to winning is to get a tower lead of 3-2 and defend it. you put your best defenders at top and bot and your other 3 roam the jungle. catching random people going for isolated caps or retreating to a tower if a 5 man push is coming on one of your towers. a good defender + 3 teammates and a tower should be able to hold. meanwhile your other defender can push the tower on the opposite side of the map.
if you compare it to summoners rift, dominion is full of "split pushes" but the game is mostly 5v5 since the respawn time is so low so you shouldnt really be worried about fighting man down. you should be able to hold a tower with 1 less teammate than the attacking team as long as your team is balanced. i see teams with the tower advantage lose all the time because they tried to take more towers, then died while the other teams distractor stole the middle tower, and the surviving members that defender the tower take top.
some characters i think are really damn good in dominion (and their roles):
akali, obviously- top tier at everything. can escape easily and slow chasers being a great distractor. burst giving great attack and defense skills.
poppy- similar to akali in dominion sense. move speed buff makes for great map control and distractor capabilities while huge burst makes her good at def/atk
shaco AP- best defender, decent distract with move speed, blink, boxes for mapcontrol, easy escape, but ****y atk on towers.
shaco AD- decent defender, decent attacker, and same goodness for distracting
most melee dps (trynd, jax, xin, trundle, olaf, ect) mostly beast attackers, pretty decent at defending but can get burst down, and pretty bad distractors. but if your team has the advantage they can be amazing roamers taking down anyone trying to cap a random turret or retreating to the tower to hopefully not get bursted down as a group
heimer- good def all game, decent atk late game, pretty bad distractor
stealth champs (twitch, eve) much better in dom than in SR. mostly because of roaming and distracting capabilities. eve doesnt really have to do **** besides make you retreat from your defending position and let your team come in. even if she dies, it doesnt matter; you lost a tower.
fast, tanky dps or casters (udyr, singed, mundo, leesin, GP, irelia) beastly champs. can usually attack and defend well with some cc and some damage, as well as if defending survive long enough for your roaming team to help. gp is particularly beast due to his ult. and amazing distractors. multiple enemies will have to come stop you from caping random towers because you'll usually just kill one of them.
random point: kills dont matter. dont chase. cap towers.
global ults (panth, TF, noc) hit or miss. i think panth is the best of the 3 building as tanking dps for the disrupt, atk, def goal, but ive seen tanky dps nocs **** and move speed AP tfs **** as well. good pics especially in draft mode when you can counter the enemy team. can fit all roles.
support champs - much worse with the heal nerf but ive seen jannas do really well with all her cc, shield, and move speed buff for the whole team. janna or sona are probably the only 2 supports i would recommend and mostly because of move speed buffs. then sona can nuke a bit while janna can shield and give 50 extra damage.
true tanks (mumu, shen ect) decent defenders and support for atks but i think there are better choices for champs in dom. rarely is there a team fight of more than 2 or 3 people which is where you really need a tank for cc and initiation. kinda like how you rarely see true tanks on TT
ranged dps- decent atk and defense but dont excel at much. probably the best bet would be a disrupting phantom dancer ashe if you were going to play ranged dps. run fast, cap distract, slow, run away. but i have seen a few ranged dps do well. maybe just against bad teams though. dont see much value here
hard caster- tricky role. casters usually do best going mid in SR and getting lvls ahead and dropping people. everyone is pretty much always on the same lvl with money and lvls in DOM so they lose the advantage, but they can still be useful. burst is one of the most important things for atking and defending. i dont think this role us useful for distracting at all but a caster in good hands can definitely fit into a team. i wouldnt go all caster, but if you had one for sure good defender and one for sure good attacker, a good caster could fit in nicely.
there are also many moment specific decisions that must be made. lets say a team is ap shaco, hemier, akali, singed, and trynd. you have 3 turrets capped, shaco is top, heimer is bot, and the other 3 are roaming mid defending from random attacks to mid and watching to look for a 5 man push to any turret. if the 5 man push comes to top or bot they join the side, but what if the 5 man comes for the mid? do they defend 3v5 and lose while heimer and shaco try and take the other turrets? or does heimer or shaco come down to mid to help? it also depends on how the minions are pushed but in general i would say heimer would got to help while shaco would move into the middle and watch for the enemy team to change to another turret instead of trying to cap more. i generally feel if you have 3 turrets its better to properly defend them than to try and take more. a rookie mistake alot of players fall into.
ill probably edit this later but feel free to comment. im thinking about posting it on a forum but ive been drinking so some parts may seem confusing or just not explained thoroughly enough.
after writing this i googled a guide to try and figure out where to post it and something incredibly similar has already been written. now i feel like i wasted my time. incredibly similar with chars being labled "attackers" "defenders" or "ult"
http://na.leagueoflegends.com/board/showthread.php?t=1286491
he also rated champs in a tier list but i dont agree with all of his ordering. mostly, akali is god tier and he didnt put her there.