No LTE.Negatives:
No visual voice mail, no tethering, no MMS. Can be resolved with Jailbreak.
No guarantee of rates/terms not changing.
I'd imagine most iPhone owners don't care today about the lack of LTE, but I see this becoming a deciding factor for many when there is an LTE iPhone.
Since Straight Talk doesn't do contracts, you're free to leave at any time, and they're free to change their rates/terms each month that you re-purchase their service. I'm hoping that they don't just drop their data cap from 2GB to 1GB, like Red Pocket recently did, but there's nothing stopping them from doing that.
Straight Talk is currently a great money saver, if you can live with the negatives.