Arena
+ Wide, open world.
+ Fairly interesting NPCs with their own backstories and a pretty decent "Rumors" system.
+ Quick, Diablo-esque dungeons make for quick fun.
- No skill system, lore, or factions whatsoever.
Daggerfall
+ http://uesp.net/wiki/File%3a%44F_Custom_Class_Creation.jpg
+ Numerous factions and commoner quests. I'd like some randomly generated quests in TES V just so you have more stuff to do!
+ Dark, complex dungeons which are both unnervingly ambient and require a lot of persistence and wit to survive.
+ The ability to fail. Quest deadlines, being expelled from guilds, assassins being sent after you. I feel like an actual person again!
+ http://www.youtube.com/watch?v=fZ97I2p4_YY
- Wilderness is horrendously boring compared to http://www.imperial-library.info/gallery/dagger_shot02.jpg%20%5bURL=http://www.imperial-library.info/gallery/dagger_shot03.jpg.
- Buggy gameplay (even with patches), adventuring can get repetitive.
Morrowind
+ Intricate, varied, hand-detailed world. I still find new things now and again.
+ Amazing lore, storyline, and general immersion in the world.
+ Excellent little touches, like NPCs who shield their eyes during ash/blight storms.
- Clunky gameplay in some respects - i.e. sneaking, magic, and even jumping (terrible collision on those railing posts!).
- Soundtrack I found to be fairly repetitive aside from a few exceptions (the theme, Knight's Charge, and The Road Most Travelled).
Oblivion
+ Radiant AI. It leaves much to be desired, but I was still amazed to see NPCs not standing around in one area all day!
+ Combat was vastly improved - I didn't mind Morrowind's system, but overall I enjoyed the fluid animations and ability to block voluntarily!
+ Traps that were more than fireballs exploding in your face, more manipulation of the environment such as drawbridges and metal gates.
- Incredibly boring world to explore, soulless NPCs, boring dungeons with no variety.
- The storyline. It's not that it's cliche'd, so much as its urgency and general mood could've been conveyed better - there's Daedra tearing up the countryside and you still come to the pub everyday to talk about your experiences with Mudcrabs?!