I played the game to completion with the levelling mod that Stark pointed me to, and I have a few more observations--
1.
This plugin makes play way, way more organic, sensible, and fun. I highly recommend it to anyone who plays the game. All the observations below are based on a game played with this version.
2. Assassins rock the house down. Their combination of bows, blades, sneaking, and alchemy is a serious threat to any enemy you'll run across. With the right planning you can run some very fun scenarios.
For example, I got a strong alchemy skill and made sure to always keep a stock of certain poisons--my most used brews were chameleon, paralyze, and silence potions. The Spider Daedra that gave me so much trouble before became child's play, because I could isolate them and smack them with a silence-poison arrow, leaving them capable only of running up to me and punching, which they aren't very good at. Paralyze was a godsend in melee combat, especially against Xivilai and Dremora.
Also, there's equipment. I did Meridia's quest to get the Ring of Khajiiti, which has a 25% Chameleon enchantment. Later, I got a sigil stone with a 30% Chameleon enchantment that I laid on a pair of blue suede shoes

. With my high sneak skill in conjunction with these two items, I got through most Oblivion Gate runs without even tickling an enemy. (If I felt I was in danger of being spotted anyway, I quaffed a chameleon potion--cumulatively making myself only 5% visible at lower levels and effectively invisible later.) Sneak past dozens of daedra and grab the sigil stone--job finished. Bitches.
3. Occasionally in the game you'll hear people say something about the Nerevarine (your character in Morrowind), to the effect that immediately after the events of TES:3 he went on an expedition to Akavir and nobody knows what happened to him. Can you say "copout?" I thought this was an extremely annoying way to cap the Morrowind story. Basically, the champion of Azura was reincarnated and set the Tribunal's betrayal aright, in the process killing at least two "Gods" (and maybe three--I always killed Vivec after finishing everything else, which if anybody who has passing familiarity with TES lore wants to ask why, I'll explain later) and destroying one of the most powerful artifacts in Tamriel, immediately
abandons Vvardenfell for some expedition to Akavir, which I don't think was ever referenced in any way during the Morrowind game.
I think that that trivializes what the Nerevarine accomplished during TES:3 and is extremely annoying--I think the writers realized that the presence of a living Saint Nerevar in Morrowind would have serious consequences on the state of Tamriel in the game lore, and preferred to just not even deal with it, so they handwaved it away.
4. Your allies are all assholes and should fuck off. Seriously, they're helpful in a sense, because they do damage to the enemy and offer another target, but they
constantly get in your way when you're fighting. I don't think I ever had a combat with allies in which I didn't stab them several times because they blundered into my sword.
5. The Morag Tong was way, way cooler than the Dark Brotherhood.
6. Enchanted weapons are handled stupidly. They run out of charges rapidly, the costs of recharging them are prohibitive (1 septim per point? Are you fucking kidding me?), and if you're the type of character who has the soul gems to recharge them yourself, them you're probably not getting very much use out of enchanted weapons anyway--because you're a mage, after all.
7. Vampires in TES games are handled better than in any other game I've played featuring them. They're grotesque and frightening and never fail to freak me out a little when I fight them. I felt the same way about the Sixth House enemies from Morrowind. They managed to evoke my horror better than most enemies in most games.