I recently got all released fourth edition books as a gift and I have been looking over them and noticed that the essentials classes feel like they came out of 3.5 and the third players handbook feels lackluster overall. Does anyone have houserules/errata that fixes this?
Do you mean other than just not using them, because that works fine, IMHO. Psionics & Monks aren't for every campaign, and the Seeker & Runepriest are pretty odd/specific concepts. I suppose the Seeker was a nod to the 'spellcasting ranger.' The Essentials Classes can be safely ignored, as can the flood of stupid 'align to the classic game' errata, if you can untangle it all.
Things that /are/ salvageable from Essentials include the skill and, especially Skill Challenge, rules from the Rules Compendium, and the MM3-style core monsters reprized in the Monster Vault.
The HotFL/K Essentials Classes are intentionally imbalanced to call back the failing of the classic game - like 5e writ small. The martial classes are designed to be optionless and strictly inferior outside of beatstick DPR; the Mage to power up wizards with OP encounters (1/2 damage tacked onto many of them) to go with already OP dailies, including spells that work automatically no attack/no save, just, push button, stuff happens to the enemy, and with spell preparation that makes them the most versatile class in terms of powers, period; Rangers return to being low-grade casters. Most of the rest of the Essentials classes are meh-to-OK 'simplified' takes on their respective parent classes.
There's a theory that Essentials was intentionally designed to ruin 4e. It's a typical conspiracy theory, but, as they often are, understandable.
I already banned the dragon magazine features since its impossible to find anything. I heard a few rumors about the seeker class being horribly imbalanced and that the warlock has issues, but I also heard that the runepriest is nearly unplayable. Anything else I should know?
The Seeker is just under-supported because of the Essentials half-ed rev-roll. Same with the Runepriest. Psionic Power brought the psionic classes up to snuff, even if it didn't fix all their issues - they're still too prone to spamming the same augmented power, for instance.
You could run very good games keeping to 'Core' PH1 & 2, only. Divine Power really helps the Paladin, and the Martial Power books add a lot of interest to the Fighter, particularly (the ranger stuff is a tad meh), so 'Power' books can be a good edition. On the DM side, you could do well to tweak MM1 monsters, in particular, using MM3 as your touchstone, definitely has the best monsters.
Some good/common variants include turning inherent bonuses on, so you worry less about magic items, and tossing out the Expertise 'feat taxes' (either completely or giving the bonus for free, depending on how you want your campaign to run at higher levels - it's more consistent with the expertise +3 bonus at epic, but more dynamic without it, turning more on crits, buff & dailies than on damage-grinding).