The skill tree isn't unbalanced, it's just so big that you can easily mess up by not picking the right balance of damage/survivability, or missing out on powerful synergies. A character with lots of +good nodes will be okay, but it will never reach the potential of a well planned build that sets up the perfect synergies to match your choice of attack and defense skills.
In general, you want to dedicate the majority of your points in the passive tree to survivability. Damage scales well enough off skill gems and weapons, while survivability requires +HP/+ES/+mitigation nodes from the passive tree to reach acceptable levels.
The notable "gear checks" off the top of my head, in no-spoiler format:
- Mid Act 1: HP/Physical mitigation
- End Act 1: Cold resist
- End Act 2: Lightning resist, HP/Physical mitigation
- All Act 3: Lightning resist
- Early Act 3: Chaos resist
- Mid Act 3: Fire resist
- End Act 3: All elemental resists
On top of that, rare mobs can represent gear checks at any point. In all leagues, watch out for reflect aura mobs. In Nemesis, watch out for Corrupted Blood, Volatile Elementblood and certain combinations with Soul Eater or Fractured.
In general, Normal just requires HP and smart play, while the resist checks above become increasingly relevant for Cruel and Merciless. You probably want to get 75% all resists ASAP - physical damage is a lot more predictable and doesn't cause nasty status effects.
The game can be extremely bursty. If your character feels sort of vulnerable in non-extreme situations, you are almost certain to die when a you reach a particularly nasty boss or rare mob, unless you know exactly how to avoid the danger. For hardcore, you really should feel safe 99% of the time to have a build that can survive the outlier cases.
By the way, I HEAVILY recommend playing hardcore, specifically Nemesis. The game is an endless grind at high levels, which can be terribly boring in softcore. In hardcore, you'll find more excitement from your near-deaths, and the occasional death will let you return to what is imo the most fun part of the game - building a new character, with slightly more resources and knowledge than the last one.
If I was starting as a new player, I'd probably play softcore until you finish Cruel, just to get a feel for what is dangerous. Then abandon that character and move onto Nemesis. You can always return to the softcore character to practice bosses or whatever.
A final note: A lot of builds will NOT work well until you are high level and have specific passive skills, all support gems, and gear with plenty of links. For leveling 1-60, most people choose some basic skills to spam while building their passive tree towards their end game build. For instance, if you are planning on making a summoner, don't start packing your gear full of zombie/skeleton gems and expect to level fast. Summons require lots of support gems and passives to be good. Instead, put e.g. Freezing Pulse in a piece of gear, slap LMP, Faster Projectiles and Faster Attacks on it when you can, and level with that. Collect summoner gems, and transition somewhere in the 40-60 range.
Popular leveling skills:
Melee weapon based:
Leap Slam (Double Strike/Heavy Strike for single target)
Cleave (Double Strike/Heavy Strike for single target)
Ground Slam (Double Strike/Heavy Strike for single target)
Spectral Throw (Double Strike/Heavy Strike for single target)
Bow based:
Rain of Arrows (Frenzy for single target)
Lightning Arrow (Frenzy for single target)
Spell based:
Freezing Pulse (Bear trap for single target)
Fire Trap (Bear trap for single target)
...and I probably forgot some
Basically, if it works well at level 10, you can do it until level 50. If it doesn't, it probably requires a high level setup.
