Gonna be hard for me to be objective about this because on top of enjoying the game, it's also deeply rooted into my nostalgia. I think we all know it's quite easy to say 'bah, that's not even a flaw worth mentioning!' when something reaches that status
But I'll try!
One of the more impressive parts of the game, I think, were the live-action cinematic graphics. When you went toe-to-toe with monsters and such, you could really see how nicely they looked and were animated (or captured, I guess). The rest of the game looks pretty pixelicious and smudgy though. Even for a game that game out in 1995, the world graphics looked quite dated, maybe even indecipherable in some parts.
The movement is pretty basic and may feel restrictive and clunky at this point, since you're limited to only the four cardinal directions. It's alright for this game, since it was designed around it. It may not appeal to somebody who is just jumping into the game 15 years later though. Think Dungeon Master or Anvil of Dawn.
The game can also be quite difficult (personally, a plus for me) in terms of combat and puzzles. I'm not going to lie... back when I first finished the game - jeez, a good 11 or 12 years ago - I had to use a walkthrough for two points in the game because the clues were quite ambiguous and I couldn't figure it out. Even from a recent playthrough, I had trouble with one of those two and it took some memory-wracking to figure it out. Getting through the difficult parts of the game is quite rewarding though, as it is in most games.
Melee can be very challenging. It involved pointing your mouse at a part of whatever you wanted to hit, and clicking. Your guy would attempt to hit that spot based on his skills. It could be called weakly strategic because each monster has weak spots that you have to find on your own.
The safest way (but maybe the most boring, but at the same time I was more interested in exploring and finding out what was next) was to used ranged items either by throwing sharp objects or shooting a bow or crossbow. Enemy AI was alright for fighting (they'd throw stuff at you and heal up if they were hurt, maybe even cower in fear or run away for help), but they can't walk through doorways, so you could sit in a doorway and just shoot through it while they stood there and died.
At some points the monsters would run to a safe place away from a door, but for the majority, you could wipe them out this way. That strategy doesn't work in the last levels though.
Besides that though, combat is exciting because you know it's dangerous, and the combat music chimes in to make it a tad more epic.
The story and voice acting were also a plus. It's a fantasy game with your standard dwarves, elves, even faeries (hilarious point in the game, by the way. There is some good humor to be found and it still makes me laugh.). Typical good versus evil plot. I guess what makes it enjoyable is the voice acting and the way the story/game plays out. You pick up tidbits from scrolls and NPCs you run into, and the places you visit are fun to explore.
If you read about it on wiki, you would know then that the opening cinematic cost them a ton to produce

. Back then it was amazing, right now it can be pretty cheesy.
The opening 10 minutes of the game pretty much set you up really well, and I think this little video gives you a decent idea of what the game looks like and feels from the get-go:
[youtube]http://www.youtube.com/watch?v=ZuPkbMFiWh8[/youtube]
There's also a 120-video series on youtube of a guy playing through the entire game from start to finish.
As for being an RPG... I wouldn't really call it that when it came to gameplay. You have stats that increase and stuff based on how you play, but it's pretty basic. NPCs join your party, but you have no control over them, and they aren't very bright. They probably are one of the more frustrating parts of the game. The setting is certainly from familiar RPG territory. The atmosphere is very good, and you are always convinced that you are in a dire situation and dangerous world. Lots of cryptic messages, objects, scenery that reassures you that you need to be careful about your next step.
I'm sorry, that really came off as a review. I guess it's hard to talk about something concisely that you have a well-formed opinion on with lots to say about it

. But you did ask, so I tried to be as descriptive as I could be!
There are Stonekeep forums over on GOG where you can get some impressions from newer players, both good and bad.