Yes, the I Fund is missing, what, 30-40% of current world economic growth by excluding emerging markets; it also excludes Canada, by the way, and ALL foreign small and mid caps. The new Vanguard FTSE All-World index and VEIEX can help balance this, but they both charge small purchase fees. PRMSX is a very good actively managed emerging market option, and with the high valuations of emerging markets as a whole right now, I think an actively managed fund is the way to go. PRIDX isn't a bad smaller cap foreign option.
Someone mentioned REITs; they are included in the domestic fund indexes -- a lot of them are mid-caps. I doubt there's much real estate exposure in the I fund; most aren't in the mega-cap territory of the EAFE index.
Another I-related TSP gripe is that the L funds are too heavily weighted toward the S&P 500, with not enough exposure to international or mid/small caps.
The TSP by and large is a dinosaur. When it's time to retire, unless there are big changes and more options by then, I'll be pulling out and transferring to Vanguard and T. Rowe Price.