And a comment on your Oblivion issues; there came a point where I realized fighting was pointless. From then on I just blitzed every oblivion gate; ran past every fight just shielding and healing til I got the sigil and that's a win.
This, almost exactly. Enchanted a bunch of clothes with Shielding. Had the ring of Skimming, and a couple fortify speed/athletics spells, along with a regeneration over time spell. I think my record for closing an Oblivion gate was about 2-3 minutes from entry to grabbing the sigil. It just was NOT worth it to kill all the dremora and elementals and what not. I ran so fast, and just blitzed past everything to the point that unless I hit a dead end, nothing had a chance to attack me.
Kinda glad Skyrim didn't give us this many "tools" like Oblivion did. As for the trifecta of Alchemy-Enchanting-Smithing, I refuse to do it out of principle. Same goes for stun-locking with Destruction (still use the stun, but not abusing it). Shield Bashing I use (but not abuse), because it is just good fun and hilarious to stop a Dragon mid-breath while yelling "NO! BAD DRAGON!" at my TV.
I can play Skyrim with a level 49 character that has the maxed damage threshold and 50% magic resistance, and most enemies are absolute cake. But every so often, out of nowhere, I find myself fighting a Draugr Deathlord or an Arch-Mage or an Ancient Dragon and with one hit, they will absolutely THRASH my HP, despite the 85%/50% damage reduction and the fact that most of my stats went to Health. With the dragons and the arch mages in particular, there's not so much I can do as the dragon will just fly by breathing fire and if there's no place to take cover, I have no choice but to try and heal up. With the Arch-mages, their damage output is often even worse since they just send a constant flurry of hits that take well over half of my HP. Before long, I simply find myself spamming HP pots until I find an opening of some sort, OR the Sanguine Rose is an absolute neccesity for my combat characters, just to draw focus away from myself. I do play on Master difficulty, so of course it is worse, but even so, I'm referring to the DRASTIC difference in enemy strength. On one hand my armor and magic resistance makes me almost unkillable by most enemies, but on the other hand, I feel like with the stronger enemies, my stats don't matter at all OR I would immediately die if I somehow accidently unequipped a single armor piece. While a challenge is nice, the gap is waaaaay too big, and unless your character utilizes absolutely EVERY skill, (block especially seems to make a HUGE difference in character power, with sword and board characters feeling worlds stronger than others) fights quickly turn into HP potion spam. What do you think of Skyrim's difficulty curve? Is it better than past titles? (Morrowind, Oblivion, FO3, FO:NV) Worse? I can't help but feel like this is a repeat of Point Lookout, where in order to provide challenge, they've provided us with boss enemies (which, there's always at least one per dungeon, often more) that pierce directly through all of our stat points, negating our hard work in building our characters.
I find with the harder enemies, (even as you're levelling) you need to think and plan your attack, and sometimes consider doing something you wouldn't normally. Is stealthing up an option? (Muffle is a cheap low level spell that can be cast with basically no points in illusion if armor/skill is an issue, alternatively Invisibility Potions) Do you have the charge perk for your two-handed to close the gap? Do you "circle-strafe" and "fool" mages? (Running towards them in a curve, then just before they cast suddenly move to the opposite direction) Do you have any duration potions that will give you a buff, maybe giving you the edge? Do you have resistance potions? Can you poison an arrow with paralysis, shoot one at the enemy, then run in to close the gap before they get back up? (Doesn't take much skill, other than aiming properly, bonus if archery is a secondary/primary skill of yours) If your restoration is being used and levelled, can you charge mages/stop dragon breath with Wards? Are you watching the Dragon as much as possible? (They usually have tells as to when they'll swoop in. I usually sprint perpendicular to their approach if there is no cover. Combine this with the charge power attack to cover more ground)
This is not criticism, just some alternate viewpoints that might give you ideas of how to surmount these challenges without potion spamming, and staying in your character's "character" as much as possible.
I agree however, that Block is practically God-Mode if you play it right. Nothing can touch you, and if they get within Shield reach, they generally die.
Overall I think Skyrim's difficulty curve is a bit of an improvement over previous titles.
Also, Two-Handed users are inherently weak defensively. Your only defense is more offense. Complaining about having defensive gaps, when you purposefully chose the route of the Barbarian seems a bit silly, no?

You either need to fill those gaps with other skills (conjuration, restoration, archery, etc), or use items and your current skills more creatively to circumvent obstacles. ESPECIALLY on Master.