Yes, you're missing something. Straight Talk doesn't buy the 4G bands, so your iPhone won't be able to connect to 4G service.
Adding to my earlier answer re: iPhone 5 on Straight Talk...
Of course, there's other carrier I neglected to mention: Verizon, who is also building out its LTE network and claims to currently have the largest 4G LTE coverage in the U.S.
Straight Talk uses all the major carriers towers for their coverage, but which exactly you're using depends on which exact phone model you get from Straight Talk. Again I'm not positive which type of SIM is in the iPhone 5 you have but it could very well be using Verizon's network towers. If the back of the box mentions CDMA, it could either be Verizon or Sprint.
IF its a Verizon variety iPhone 5 and SIM...again, of course, you'd have to be in an area where Verizon has 4G LTE coverage, AND Straight Talk would have to buy access on those 4G LTE towers and not just Verizon's 3G spectrum.
If its a Sprint variety iPhone 5...same deal. Sprint is also building out 4G LTE but its in earlier stages than Verizon, and don't know if Straight Talk has access yet to any of those new towers. I doubt it though, and any reference to 4G I've seen on Straight Talk's own website refers to T-mobile 4G.
I haven't tested this new iPhone 5 deal on Straight Talk from first-hand experience, but I help sell phones and have read a good amount on this topic...
First, 4G isn't 4G isn't 4G. There are different "kinds" of 4G networks.
The new LTE (Long Term Evolution) brand of 4G data network is what the major carriers are all eventually moving to, and what the iPhone 5 supports.
T-mobile was early on the 4G bandwagon with what they call 4G which is very fast but is really termed HSPA+ and not potentially as fast as 4G LTE. Sprint's first build-out of what they called 4G is specifically termed WiMax.
As to if the iPhone 5 supports T-mobile's 4G (HSPA+), is a fuzzy question from my little bit of research. Likely no, unless you're in one of the 5 cities that have switched their HSPA+ frequency from 1700 MHz to 1900 MHz (Baltimore, Houston, Kansas City, Las Vegas and Washington D.C.), though there could be others by now, I'm not sure.
I don't know what actual company's network the SIM that comes with the Straight Talk iPhone 5 is on. I do know you can order ( straighttalksim.com ) a standard- or micro-size SIM from Straight Talk that uses T-mobile's 4G (HSPA+) network...
BUT, 1) the iPhone 5 takes an even smaller nano-sized SIM, and 2) The fine print from Straight Talk says you can only buy a stand-alone SIM from them to use for an outside vendor's phone or unlocked phone, not a Straight Talk (or any Tracfone-owned company) phone.
For the real details on these points, I would call Straight Talk's customer service. If the first person you get can't answer your questions, ask to talk to someone else or a supervisor.
Ask what network is used by the SIM in your iPhone 5. If its AT&T, there's your likely answer, I doubt Straight Talk gets to use any 4G LTE on AT&T though I could be wrong! If its T-mobile, then you're likely not in an area with T-mobile 4G HSPA+ 1900 MHz-frequency towers that are nearby. Of course the same could go for AT&T, you have to be in an area with that type of 4G coverage. AND Straight Talk access to those towers. AND open to the same frequency band used by the iPhone 5 for 4G LTE (1900 MHz vs. 1700 MHz)...not confusing at all, right?
And worth asking if they are or will start selling nano-SIMs for the iPhone 5 that use another network than the one you have (well, specifically, T-mobile 4G).
If you make it that far, do post back here what you find out! ;)