The lock-on system is really nothing more than a target indicator showing you what you are aiming at currently, you can change how it works a little though. If you have three mobs in front of you, but one has some sort of debuff on it, you can set the reticule to "lock" on that mob before the others. It also acts as a range indicator for skills, if you see a mob you are aiming at light up, then you know its in range of your skills.
Combat just feels more dynamic, classes have many skills that are reactive to what you do, or another player does or what a mob does. Like my Destroyer will leap to a mob, which stuns it (and everything around it), which triggers skill that lets me launch it into the air, which triggers a skill that allows me to jump up in the air and pile drive it into the ground, which stuns it again, and that allows me to grab it in a choke hold to either punch it, slam it down into the ground, hook it with my axe and toss it far away, twirl around with it to hit anything around me with its body, or launch it back up into the air again (my pile drive skill can reset on a crit).
Then you can combo off what other players do to mobs, and this is important in dungeons when you want to stop a boss from using a skill. If another player uses some sort of "prep" skill on the boss, you will see a brief flash of color around the boss (color denotes the type of skill..stun/weaken/knockdown), which will allow you to use a follow-up skill to either stun/knockdown the boss. You can setup the UI so that you get a big prompt in the middle of the screen telling you what skill to use to finish the combo (either press its hotkey or spacebar). Once you get used to the control setup, combat flows really well