Actual combat can be about as simple or as complex as you want it to be. We're fans of the classic "fighters slash toe-to-toe with enemies while spellcasters hang a couple spaces back and lob magical death" school of thought, and the fierce-looking animations and simple interface complement that approach nicely. You'll probably get a lot further, though, if you know how to exploit things like sneaking and flanking bonuses. You'll probably also want to learn the difference between a guisarme and a ranseur, because those are among the staggering selection of weapons you'll be able to choose from right off the bat. Really, the only things needed to make the classic D&D experience complete are some iron rations and a songbird.
You'll have more to worry about than just simple combat, of course. The dungeons and other environments you'll visit tend to be really big, so there are often a lot of opportunities to explore, find hidden areas and pick your way through traps and puzzles. You'll also frequently need to carry around a torch or other light source (unless your characters are all non-human and can see in the dark), which brings out one of the game's coolest features: real-time lighting that flickers, casts realistic shadows and is completely blocked off if there's a wall in its way. It's no mean feat for the PSP, and it does a great job of setting the eeriest possible mood while you're moving little polygonal fighters around on a grid.