Good idea.
I have some experience with shields and steel melee weapons in real life. Basically, there are four main powers shields do provide.
1. Passive defense: it is just "there". More noticeable with larger shields. Even if you are completely oblivious to attack, there is a chance that it will go into your shield hanging there, not into your body.
2. Active defense: what we seem to have in Skyrim. Good, important thing.
3. Shield bash: powerful outward blow, tend to throw enemy back, sometimes he lose stance and focus for a while. Usually sword attack follows in the opening.
4. Shield ram: cruel forward punch with the brim of a shield, usually straight to the face. Basically, your shield works like a giant knuckles in this move.
Shields also do have several weaknesses.
1. Most of them are kinda heavy. It's not a problem if you are strong enough, but for general population it is, yes.
2. Shields always cover parts of your vision field. This effect is not important with bucklers, but with round and kite shields it can't be ignored. You especially can't see legs and whatever goes down there.
3. When you are defending from top attack, you practically lose all vision field. And you still don't see anything down there, which opens up first bread-and-butter anti-shield "combo": feint strike to the head, provoking raising of a shield, then slash enemy to the left leg around the knee.
4. Agile enemy can use your own shield against you, if shield is large enough. Basically you can't hit through your own shield, and if agile and crafty enemy goes very close, then you can't hit him, except with bash, which is not always appropriate.
Next, I do want to say several words on possible shield types. Sure, there are loads of various shields in world history, but they tend to relate to one of these 5 types.
1. Light shield. This one is probably unnecessary in Skyrim. Basically it's light wooden frame with leather strung on it. Shields of this type been used mostly in the ancient world, and they were useless in melee (that is why it is unnecessary in Skyrim), being designed specifically as protection from arrows and other projectiles.
2. Small shield. Buckler, targe. They are ligh (although they are usually made from steel, they are small enough), require most precision and often are used as an attack weapon, in bashes and ramming punches.
3. Medium shield. Round shield, smaller kite shields, not covering your leg completely. Medium is medium.
4. Large shield. Oval shields of some african tribes, elongated kite shield, "upside down teardrop" shield, probably roman scutum. They all are massive, they tend to cover most of your body, and they are heaviest shields you can realistically use in melee combat.
5. Tower shield. This shield is definitely covers all your body, from head almost to feet. If there are no holes, you can't even see your enemy. It is usually very heavy, and it is NOT designed for fencing. Most famous shield of this type is european pavise, which has been used by crossbowmen. Actually, by their assistants: they went in pairs, one carried crossbow and bolts, and another dragged the pavise to cover them both from enemy fire, while they are holding good position on some hill or whatever. Tower shield is semi-mobile fortress, and in my opinion, there is no place for true tower shields in Skyrim.
Basically, three types of the shield are "good": small, medium and large, while light and heavy probably should stay overboard.
Now, comments about shields and Skyrim gameplay. Let's check through list of shield "powers".
2, 3 - they seem to nailed it completely.
4 - may be used as a version of 3, for example you usually ram punch with buckler and you rarely do it with large teardrop shield, it is too heavy and slow to do so.
1 - one we basically lost. And I don't really know if it is good idea or not (meaning in accordance with current game mechanic, in general it definitely is a good idea), but in my opinion it should come back. Hit location detection + large shield (probably draining lot of stamina, making you move slower as counterbalance) = you don't really have to block against most attacks coming from your front-left side. But you are not protected from anywhere else, of course, so this shield type may be useless against, say, two-three agile bandits.
Now, on weaknesses.
1.1. Weight as in encumbrance. Obvious inherent weakness of medium and large shields. Small shields don't really weigh much.
1.2. Weight as in combat efficiency dropping factor. Again, most important for medium and especially large shields. Basically, your speed and stamina must drop. Heavily armored crusaders don't really care about that, but if your character tries to be more agile, then he probably won't be used anything except buckler, and that is how it should be.
2. Passive vision impairment. I don't think that it is that important in game, because our screen provides us with much narrower tunnel vision than our eyes. Shield probably should be always visible down there, but it is wrong from gameplay point to make it annoying.
3. Active-usage temporary blindness. Now that probably is important enough, to animate shield defences so that it covers your vision, but... not really good idea. In 3rd person you won't be troubled with this at all, and it is just annoying. It should be short and it never should cover all your limited 1st person vision field.
4. Sneaky agile enemies using your shield against you. While that would be nice, I don't think that it would be in. Although... sounds like a good perk for thief-type characters. So probably it is not bad mechanic. Basically it could go like this: when your enemy, who has this perk, charges and stays close enough, he is protected by your shield just as you are. He stays "glued" somewhat, and he can choose time to break and attack. And you are annoyed and trying to bash him away, but you not always have stamina for this. More than that, when you do bash him, all it does is he jumps away, completely not wounded by your shield (if it's a perk, its' bonus should be noticeable).
So, suggested difference between shield types goes like this:
1. Small shields.- Weak, almost absent passive protection.
- Active defense almost require perfection in timing.
- Shield bash is animated as ramming punch, it is way more damaging than other shield bashes, but it staggers enemy very little.
- Low weight, stamina is not drained much by using it.
- Speed is almost not penalized.
- Enemies can't hide behind it.
2. Medium shields.- Noticeable passive protection, provide effective cover from arrows even when you don't care about it.
- Active defense is way easier, you can "turtle" with it.
- Shield bash is what it is, it staggers your enemy and damages him. It has some chance to drop your enemy down.
- Noticeable weight, stamina is somewhat drained by with every active move, and it is drained for some points even for equipping of the shield itself.
- Speed is somewhat penalized, but character is still an "agile fighter", but not a "ninja".
- Enemies can hide behind it. Shield bash helps to get rid of them, yeah.
3. Large shields.- Awesome passive protection. Most attacks from one side are ignored. If you are trying to snipe enemy who is carrying this type of shield, you'd better lay a headshot or wait until he shows his back.
- Active defense is easy. You can put it out, like in Oblivion, although it will drain your stamina continuously.
- Shield bash is very slow and not effective as damaging move, but enemy will stagger and have a good chance to fall down.
- Damn heavy. Stamina is drained like there's no tomorrow.
- Speed is heavily penalized. This kind of equipment is not compatible with various acrobatic feats. Basically, you can only turtle-and-counterattack with this shield. Fortunately, this combat style is popular.
- Enemies can easily hide behind it. Shield bash helps to get rid of them, but it is harder to even perform shield bash with such heavy boars, when there is a nasty ninja holding it from the other side.
Phew, that's all I have to say about shields. Oh, wait, another one... shield spikes. Good for bashing, bad for ninjas trying to hide behind your spiked shield. Now I'm done, yeah.