I'm going to be talking about ESO's siege mechanics from a DAoC perspective as I played the game for many years but seeing as how ESO is the only true spiritual successor (although imperfect) to DAoC's style of frontier RvR, I believe this comparison is entirely appropriate.
I'll get straight down to it. Every siege of a keep I participated in and witnesses I saw everyone and their mother throw down siege within seconds after arriving at the gates. Within a minute or two the siege limit is reached on both sides of the battle and the field is littered with ballistae and trebs. With so much siege in every battle, siege loses that "epic" and "shock & awe" aura that I believe it deserves. Given that any player can easily place down and retract siege at a moment's notice makes it such that siege can be repeatedly used and one doesn't even think twice about placing it down. Siege becomes abused and far too commonplace. And even if siege equipment is lost, it's not terribly difficult to replace. After all, for just a few hundred gold you can buy multiple trebs or ballistae.
I enjoyed DAoC's mechanics of making siege a serious investment. Once you placed down siege, it was there to stay. You also needed craftsman to help create the components and multiple players to carry them given their extreme weight. Creating siege wasn't exactly cheap either. If you wanted to bring down that enemy keep faster, you were going to have to pay for it. As a result, siege was a lot more rare, and certainly a lot more terrifying when you were peering over your castle walls only to discover the enemy had just built a treb. When you were behind a siege weapon, you felt like you really were a one man wrecking crew, it felt intense, powerful and epic.
In all, most keep sieges in DAoC never saw more than 3 or 4 pieces of siege go up. Subsequently, they were high priority targets for your enemies and something you needed to defend at all costs. It added a layer of strategic depth to the PvP that I feel is lacking in ESO. Over the last few betas, I've probably single handedly burnt down dozens of siege. In fact, siege is so immensely undervalued that people will gladly abandon their siege and flee at the sight of even a small enemy zerg to save themselves because everyone knows you can pop up another treb in no time flat.
I think that ESO would greatly benefit by making siege significantly more expensive yet also make siege moderately more powerful than it is now. Making siege truly valuable weapons of war will increase elements of strategic play as well as fostering the tremendous emotional impact seeing siege on the battlefield should inspire. I'm eager to hear your thoughts on the matter as well.