1. Level 90 Talents
On paper they are awesome for arcane, because the way they interact with mastery can drastically change the play style, but for fire or frost they are minor annoyances. In practice: RoP is frustrating, Invocation is boring, Incanter's Ward hard to use and high risk.
2. Frost's Double Cap
Keep the Shatter cap, make Icy Veins glyph baseline? Make Shatter scale over 100% crit (similar to Chaos Bolt)? Stacked on top of the hit cap, the crit and haste caps can make gearing frustrating or boring. However, frost's shitty scaling doesn't bother me if Blizzard can tweak the numbers each patch. It's not elegant but it works.
3. Fire's Crit Scaling
As I said above, adjusting specs between patches is acceptable. What's not acceptable is a spec that goes from weak or boring to a critfest in a single tier, only to be knocked right back down after the patch.
4. Arcane Play Style
I want the burn phase back. If the level 90 talents aren't removed, Invocation should enable the old burn-then-conserve play style.
5. Minor Glyphs Are Boring
Yeah okay I ran out of great concerns. Ask someone who does PVP.