I'm a big time QoT player as well, and I think she's easily one of the stronger heroes if built right. First off, spikes is one of the best abilities in the game IMO (not quite on the level of erebus' bite, but still very good) I think what people fail to take into account is that spikes doesn't just mean that you're doing more damage. It lowers their armour, so your minions, as well as the random creeps on the map, and your allied heroes are all doing increased damage. On top of that, it has decent damage straight up. This, combined with shield, gives her the best attrition of any demigod. One on one, she's got the best survivability, as well as damage decent enough to push all but the toughest eberus players back. Another important thing to remember is that you're one of only 2 heroes with a ranged basic attack. This really lets her dominate a lane early game against basically every hero if played right, because you sit back and hit them from afar.
If they aggro towards you, move QoT back and let your minions (I go for shamblers and monks) damage them on the way through. Keep running till they give up, and then turn right around and start harassing again. Her shield also lets her basically ignore UB's spit, making her very good at holding flags against agressive capping UB builds.
Part of her siege power comes from combining spikes with waves of creeps, minions and allies. I'm used to teaming up in 2v2 games with a mate of mine who plays Oak, and frankly, the combination of spike and pentinence lets you push a tower in no time flat, and can see an enemy demigod fall in even less time.
As far as build goes, I'm surprised to see so many people supporting mulch shamblers. IMO, this ability is completely redundant. QoT is quite starved for skill points, needing 9 of her 20 just to get the all important godess of thorns. Not to mention you wont often get games going past level 16 if you're playing agressively enough, so it's even more imparitive that you have the right build.
In general, spikes should be your priority. The less supported you keep enemy assassins in the creep lanes, the less they can do to you. In general, I find that Generals > Assassins when played by skilled players, because they have a lot more efficient support skills that help keep them alive, or give them the advantage. Not to mention their summoned minions can survive most nukes, and monks keep you in battle longer. The less your opponent can do, the higher level gap you get over them, as you farm the lanes. this, supported by your shield, should keep you going (given the aid of a couple monks, of course) for a lot longer than your opponent is able to sit in lane. So the three abilities you pretty much NEED are: Spikes, Shield, Shamblers. I generally max all of these as soon as possible.
Shamblers might seem 'average' but the more minions you surround yourself with, the higher your ambient damage is, and as an added bonus, the more 'sponge' you have when pushing towers, and running from chasing demigods.
Between maxing these and Godess of thorns, you have a couple of points to spend. I generally get 1 level in spike wave for the slow, if my enemies are getting away from me, or just go for entourage and/or the + stats.
Items are important to keep the queen going right from the start. Generally It's all about defense, as you'll be going for an offensive build right off the bat, with the spikes being priority, so you have to have something to keep you going. Generally I either go both of the first two armours and my ally takes monks, or I take monks. Generally I prefer the earlier, so that I can hold off for bishops later (I find their survivability invaluable for fleeing in later game, to compensate for no mulch )
The most important thing to do next is get your mana base up. I suggest either go pelnor battle crown and vinling, or just take the heart if that suits your fancy more. I think the attraction of the hats over the heart is that if you DO get caught out on low mana, chances are you'll be able to get away, whereas with the heart, you have to rely more on good prediction to get out of hairy situations. (also it's a bit more expensive). Late game, if i'm getting killed, I stack health items, replacing my early armours with Grofling and Godplate (if I have enough) and just attempt to stall the enemy by sitting on their portals, and then porting to the otehr side at the first sign of trouble or if I have a few kills under my belt, and level advantage, I stack speed (I always take speed favour item when favour is enabled). This works especially well for me because, being allied with Oak, your movement speed actually gets quite silly with surge of faith.
You have to remember with Queen of Thorns that she is a roadblock, who can be quite an issue if you ignore her. So you have to play to take the map to your advantage. If you're having problems with assassins, sit in lanes where you can easily get to important flags where assassins wont get any minion support (like the goldmine flag in crucible) and push, push, push! I can't stress enough the importance of an agressive game. Force the opponents to spend more time and resources than they want to trying to counter you, because funnily enough. Most of the time when they're actually getting somewhere, you can just run away and live to talk about it, and your ally's probrably made some good headway elsewhere on the map if they arent a complete newb.
I could go on for another 5 pages on how to use her in ganks, lanes, portals, which flags are important for you, which maps she's more suited to, and about a billion other points on why she's easily one of the better demigods (and why i'm so madly in love with her) but I'm sure it's enough to just try to get through this much. And Kudos to anyone who reads this through! I hope it helps you with your games using the Queen