Older sims are missing the USIM "applet" that contains the data necessary to negotiate with UMTS networks.

This is something weird that I've discovered too.

I am now on T-Mobile (from AT&T) and there are all sorts of T-Mobile features that may or may not work depending on what SIM card you have in your phone. And if they don't work you have to get a new SIM card (that is not guaranteed not to disable other features.)

What!? That seems to be the weirdest thing ever. What is going on?

Mine is defo new enough that it supports that! Anyone know if you need a new sim card to support 4G services?

IIRC, yes, at least for AT&T's LTE -- HSDPA (and HSUPA, HSPA+, etc) will work just fine with a 3G SIM (since they're just souped-up 3G).