Gameplay
The game takes place in full 3D. What's surprising about this is that it's all implemented rather simply - you press M to move your units, and hold down the SHIFT key to move them upwards or downwards (the tutorial is quick to teach you about this). Once you get used to it (which doesn't take too long) it becomes second nature, almost. You have incentive to do it, too - all capital ships have weaker armour on the top, bottom, and rear sides.
Combat is also handled fluidly - there's a Group Attack feature, where you can hold down CTRL and drag a selection box around your enemies. Your units will promptly go and attack the whole group - I found this so useful, I couldn't remember how I'd ever done without it when I went back to Age of Empires II. There's another cool functionality with the group function (as in, assigning units to groups with CTRL+) - unlike most RTS games, Fleet Command - a voice which provides you with updates on the battle and such - will recognise the groups; so when you assign some units to, say, Group 1, Fleet Command will say "Group 1 assigned", and continue to provide status updates ("Group 1 is under attack", "Group 1 reports victory" etc). What's even more useful is that you can set groups within groups - i.e you can assign your fighters to group 1, your Corvettes to group 2 and your capital ships to group 3, then select all of them and assign them to group 10 (CTRL+0) without it erasing the individual groupings. What this does is removes the irritation from the classic micromanagement of RTS games, making it easier to organise your units - and it doesn't remove any of the challenge.
When it comes to the long game (is Meta the word?) Homeworld's campaign has a gameplay attribute you have to get used to - a persistent fleet. This means that units you build/capture on one level carry over directly to the next. It also means you have to be really careful - winning a mission with only 1 ship and no resources left isn't going to cut it for the next mission, so you have to not only win, but win well to effectively progress. You have to be careful to conserve your units and not throw them away like Zergs, because each mission only has limited harvestable resources with which to rebuild - and you have to, at the end of each level, think carefully about which ships to construct. You don't know what you're going to encounter on the next mission, and you have to be prepared for everything. It's this, above all, which makes the game so very difficult - I've heard horror stories of people who've ragequit because they went and acted stupid by not saving very often and throwing away most of their ships on a particularly hard mission.
Story & storytelling
Homeworld's story is simple, in itself, but at the same time it's what an RTS needs from a story. There are no real characters (though there are named voices), no McGuffins - the story focuses on your people and their plight as a whole, to get Home. Hence the title. Still, I'd say the roughly average story is far eclipsed by the way it's told - there's emotion and atmosphere in droves, and you really are emotionally invested in the civilisation you carry between the stars.
Graphics
Being 11 years old, you'd expect the graphics in this game to look absolutely goddamn awful, comparatively to today's games.
... They're not. The graphics technically aren't that good, and if you zoom in close enough and are on a low enough setting you can play "Let's count the pixels!" to pass the time... but Homeworld is beautiful. Really, really beautiful - from about the 4th mission onwards (out of 16). Never before has space been such a scenic place.
Music
The music is haunting, atmospheric, and absolutely brilliant. It's my personal favourite game soundtrack, because it really fits the game - you really do get the feeling that Space is really, really goddamn huge from some of the pieces, and the battle music is catchy and very cool.
So, there you have it - why Homeworld is awesome. If the difficulty daunts you, you can find walkthroughs scattered about the intarwebz (particularly on Gamespot's "hints and cheats" section, as well as their official walkthrough). I also feel as if I should warn you - I have a friend who got Homeworld to run on his magnificent and wondrous gaming rig, but he had to change it to window mode as it tried to double the framerate when it was fullscreen - which, of course, caused much wailing and gnashing of teeth.
EDIT: Removed stuff about its cost from Amazon, since you already have it... damn you copypaste!