Barghest said:
I don't want additional quests etc, as I'm only playing through it once, so the mods recommended by aweigh seem to worth having a look at...
Personally I think gameplay mods are the most important, but it's difficult to decide exactly what you'll like before you've played the game. Mythic Mods is probably the best place to start. If you have a specific list of desires, you could ask in the official mods forum (without specifics you'll probably just get links to Mythic Mods / equivalent).
Having said that, you shouldn't necessarily view quest mods as just "additional quests". A lot of the quests in quest mods are more involved and well written than standard game quests. Choice with consequence is probably a bit more common too, though I haven't played enough of them to know.
On gameplay mods, you should be careful what you add if you're aiming for a well balanced game [you can't
preserve Morrowind's balance, since it doesn't have any, but you can improve it]. Take something like Magical Trinkets of Tamriel for example. It's got a lot of great ideas, but personally I find it unbalancing if used unaltered. For instance, take threads:
A thread gives you a semi-permantent magicka regen bonus at the expense of some penalty - e.g. not wearing heavy armor / not using slashing weapons... This is a nice idea [another item can be used to exchange the magicka bonus for a permanent advantage in e.g. a skill], but is unbalancing, since you can usually pick a penalty that doesn't affect you at all (e.g. renouncing heavy armor when you never used it anyway), and the bonus you receive is too good: 1 point per second magicka regeneration is very unbalancing for low level characters.
I'd still suggest using MTT3 (or MTT4) though, since the ideas are very good. Just be aware that it'll make the game easier, so you should use other mods to add a bit of challenge.
I would suggest my own main mod (Galsiah's Character Development), but if you don't like it you've gained nothing, and if you do Oblivion will be disappointing in this area. I wouldn't suggest such extreme alterations without trying the standard system first in any case. You really need to have played the standard game a bit to make an informed decision on a mod setup.