I like the two add-ons pretty much equally (less than the original game, however), but I reserve the right to change my mind after I finish Tribunal (so far, I've finished the original game and Bloodmoon)
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Things I like about Bloodmoon...
1. The setting. Solstheim is relatively small, but has just enough variety in terrain to be interesting (for a while). All that reddish wasteland and duststorms on Vvardenfell get boring after a while (though the snowstorms probably would too...).
2. The ice. I dunno, it's just kinda neat to have ice, even if it's cheap permanent-ice.
3. The East Empire Trading Company quests. It's neat to see the colony grow, even if some of the later quests are buggy and easy to break entirely.
Things I don't like about Bloodmoon...
1. The main storyline. Well, it's not terrible, but it is kinda lame.
2. The repeatedly-generated smugglers and reavers. It's fine to randomly generate enemies in the world, but always generating the exact same set of people in the same spot gets old very quickly - like, immediately. Plus, it's a really cheap way of handling the whole lycanthropy thing - you don't have to feel bad about having to kill people as a werewolf, because there's always a ready supply of people whose only job in life is to die.
3. The balancing. Bloodmoon is poorly balanced - obviously, the difficulty is balanced towards players who have already finished the original game. Unfortunately, however, the quests are more appropriate for beginner players. There's not much fun being a werewolf, because your uncontrollable bloodlust is perfectly controllable - your character has enough hitpoints to ignore the hitpoint-loss incurred if you don't eat people. Furthermore, the Thirsk mead-hall thing is silly, because the money you can earn from it is peanuts - what's a few extra coins when you've already bought everything you could ever need? Similarly, only an obsessive (like me) would bother to loot every barrow in the game, because the items you can find are not good enough to use, and there's no point selling them for money (since you've got so much already).
4. The setting. While I like the terrain and such, there's simply too many bothersome enemies around (especially those to-be-killed-thrice things). After a while, I simply stopped bothering with them, and resorted to a constant diet of levitation potions (but hey, look on the bright side - at least there's none of those annoying cliff racers out on Solstheim).
5. All the new items. They're
all ugly, and the special snow wolf/bear armour most of all. Gimme one of those royal guard armours from Mournhold any day.
6. The weather. I
like having cold weather in the game. I like the snowstorms and such... but it bothers me that I can swim from Vvardenfell to Solstheim, then run around on the island for several days in constant snowfall, and never even catch a cold. They should have implemented some sort of cold-related disease.