The Good: Nice HD upgrade, clever level and puzzle design, leading Yorda around is a great mechanic
The Bad: Janky animations and mechanics, Yorda's AI is really dumb, not much of a story, can be beaten in 4 hours, too much backtracking
Ico is probably one of the most forgotten games in history. Being one of the first graphical powerhouses for the PS2, it proved that you don't need a complex story and characters to have a good game. All you know is that you are a boy named Ico who is trying to save a girl named Yorda from her evil mother queen and escape from the castle. You enter as a captured prisoner because you have horns. You then escape and find Yorda along the way. What made Ico such a big deal was that you led her around manually by holding R1. This forced you to become attached to her, and they rarely spoke.
This game was way ahead of its time. That's probably why people passed it by for Jak & Daxter and other PS2 games at the time. The game consists of some pretty cleverly designed puzzles that involve pulling switches, climbing, cutting ropes, and swinging on chains. You also push and pull the occasional block. The level design is really well done, but there were a few obscure puzzles like jumping off of a chain to knock a bridge down. This is completely against the game's mechanics so you would never know to do this. Leading Yorda around sounds annoying, but it prevents you from having to rely on the already shoddy AI. The game mechanics work well enough in design but they are sloppy and frustrating to deal with.
When you jump Ico tends to clip into ledges and ladders making you have to slightly adjust him until he latches on. If you press the analog stick just slightly and jump he will jump 20 feet causing you to jump off ledges or fall down to your death. There's just so much bad collision detection and the animations are janky and not well done. When you drag Yorda around her arm looks like a flopping noodle that can go through her body. I know this was originally designed for PS1 before the game jumped ship to PS2...but still. There are other mechanics like setting your sword down to pick up a giant stick, light it on fire, then set off a bomb. Some times you have to toss these before they blow you up. Fun game mechanics and all but they repeated through the entire game making you be grateful for the short length (just about 4 hours).
A lot of times I didn't know where to go because the middle of the game has two areas that are identical, but just slightly different. The constant backtracking can confuse you requiring a walkthrough. There are no boss battles to speak of except the final boss, but the combat is so frustrating and annoying. Ico swings his sword around aimlessly and you can't move while doing so. The AI jumps away as soon as you run after it so if you get stuck in a three hit combo you get knocked down and Yorda gets dragged away. You can't die in this game during combat, but if Yorda gets dragged underground you're done. You can pull her out of the hole, but some times a shadow creature will hit you and knock you back then you have to suffer the long animation of getting up. A lot of games had these animations (The Mark of Kri) and I don't know why. It breaks the fluidity of everything. Once you get the hidden mace in the game combat is really easy, but you still get annoyed by the creatures constantly backing up when you approach.
The graphics look great with the new HD upgrade, but even back then they were amazing. A lot of textures have been slightly upscaled and some bloom and HDR has been added for better lighting. You can still notice the game's age during up close shots of areas. Textures are muddy and so blurry they don't even look like what they are representing. The main thing is that you can pan the camera around more now that the resolution has been bumped up making navigation easier. This is how you do HD upgrades right.
Overall, Ico is a great game, but the dated mechanics really show their age. If you can look past all this you are in for a great, albeit short, game but don't expect engaging characters or a complex story. There are 4 cut scenes in the whole game, but this is about the journey and not the story.
(Based on PS3 version of the game)