- Unlock the next level of talents (not the final tier)
- Provide Level 70 Characters with 1 extra talent point
- Make class spells/abilities of up to Level 75 available (edit: this was originally suggested after looking at a bad Wrath database and might have the most issues and biggest change to gameplay, but should still work OK)
Also, keep in mind that all players still have total empty glyph slots, final talents are still unavailable, profession bonuses are unavailable, and almost all of the higher rank and wrath-level spells/abilities require a level above level 75.
- Protection paladin:
Divine Plea ability will be available, improving mana sustainability. Shield of Righteousness for threat. 1 extra talent point will also make it possible to get a little more seal damage or movement speed since Paladins have no charge/gap-closer.
- Feral Cat:
Feral Druids will finally get Savage Roar ability, the druid equivalent of the combat-point finisher ability Slice & Dice that Rogues have had since vanilla.
- Shadow Priest:
Boosts Shadow Priest ability to gear with and benefit from spirit through an unlocked talent, allowing for more Mp5, and buffing spellpower more so as to benefit better from Shadowfiend. This should help a lot with mana, especially as gear improves, just like in TBC (where mana came from Vampiric Touch, which was totally different in TBC patch).
- Feral Bear:
This is a little complicated, but significant. Savage Defense--the replacement for removing armor from offpieces after TBC--requires crits for decent uptime, and a steady source of these crits is Lacerate critting (in both single target--especially fast-hitting bosses like Prince that rapidly use up Savage Defense--and AOE situations), which becomes available through Primal Gore talent (this combines unlocking a level of talents and also granting 70s 1 extra talent point). Maul will also do more damage on mangled targets, allowing Druid tanks to weave mangles in between swipes so that swiping on aoe mobs while tabbing and mauling will provide threat that can scale better with gear.
There are some more examples, and of course every class and spec will benefit from this, but it will help balance hybrid classes and specs and allow for gameplay in TBC that was balanced by Blizzard in wrath (the whole point of this server), but not intended to be balanced before 80.
While this will obviously change gameplay tuning, T5 is coming very soon and most T4 content is still incredibly difficult for anyone not in one of the few top guilds. Gruul, Maghtheridon, and the later bosses in Karazhan should still be challenging for raiders even if they are decked out in preraid bis.