Offense in WSG means "getting" - getting their flag and bringing it to your flag room, and getting your flag back whenever they take it.
Offense in WSG has nothing at all to do with killing. Lots of people play WSG as a kind of dueling ground ... but that's not what we're talking about here. The goal of the game is to capture flags.
Getting the Flag
There are two basic planned flag grabs: big group and small group. Sometimes you will have to make an emergency grab ... to keep the other team from capping, for example ... but usually you want to go in with a group.
Big group: this is a massive power run ... get the flag and take it back to your place. You use this approach when the other side has enough people in their base to defend against the smaller grab.
Small group: if you can use part of your force to tie up the other team and send a small group after the flag, you usually come out ahead. For one thing the other side might be so busy that they can't get to your flag. For another, if you already have them engaged, your flag runner can go around them.
Running the Flag
After level 19, everyone has a mount. You can argue that this makes WSG less fun, but the fact is that if you are running the flag, they will catch you. This means that you have to run smart rather than fast.
Stick together in a convoy. Helpers are a good thing for a flag carrier, so don't outrun them. Slow down or stop if you need to. (Exception: if your helpers are fighting and you can run to safety, go for it.)
Heal yourself. If your health gets down, stop -- eat, bandage, drink ... whatever. Take a little time to get to full health.
Use the map. Seeing where your guys are often can tell you where the enemy is. Sometimes you want to go at your guys, sometimes around them. But you need to check the map so that you can decide.
Have a plan. When you enter your base, make a decision about where to go: roof, fr, gy, etc. ... don't just run around and around.
Communicate. You are the key the what your teammates are doing. Tell them where you are and where you plan to go.
Try to stay in the flag room. Unless you are near death, stay in the flag room, on the roof where you can jump down to the fr, or in the little room on the second floor. When your offense kills the efc, you may only have a few seconds to score before someone on the enemy team grabs your flag again. So stay close.
Convoy
You need a flag carrier, a healer, and a few dps/cc. Any combination can work. Any class can run the flag. Priests and mages make fine flag carriers. It's more about knowing how to play your character that about any particular class identity.
If I had to pick my favorite, it would be a feral druid flag carrier with a prot Pally healer with mages and warlocks for cc. Druids can shape-shift out of physical traps while Pallies can dispel magic/disease/poison as well as heal. Bear form can absorb a lot of damage, as can prot Pally. And the combination of freeze, sheep and fear is very nice for clearing a path.
Getting the Enemy Flag Carrier (EFC)
Getting your flag back is the key to a quick win. Defending your fc is nice, but sooner or later the enemy will break through. It's usually better to go after the efc than to guard yours. (If you're the stronger team, you don't want to give them a chance to score. If you're the weaker team, there's no way you'll hold them back ... so go for the quick cap.)
The first player to find the efc should call out location. Repeat this if efc moves. Yes ... the flag is on the map, but the map sometimes is hard to read. Saying "roof" or "fr" gets your guys there faster.
Be at least loosely grouped. Most of the time you don't have to wait until you're all together ... but you don't want to arrive one at a time with many seconds in between. Look at the map and if there are people behind you, maybe you should wait for them.
Be ready to grab the flag. Especially in a melee, another opponent may pick up a dropped flag. If the efc's health gets low, get ready.
Graveyard Camping
If the enemy is successfully camping your grave yard:
you are going to lose ... why are they so much better than you are?
leave a lock or hunter there to harass them and send everyone else out the other side ... get their flag and they'll have to come find you.