Although Thai AIrways is not the best Asian carrier, its seats and service should be ahead of Northwest.
TG's Boeing 747s have the 34 inches seat pitch, which are much better than Northwest's 32 inches legroom. The only bad thing about TG's Y seats is the 3-4-3 ten abreast arrangement featured on its Boeing 777 fleet.
About meals, TG still retains the hot towel service and its catering kitchen in Bangkok is not bad. Their menus do not change frequently, but the quality should be decent.
Entertainment -- both use the mainscreen... and both carriers do not have plans for personal TVs in Y class.
Mileage -- NWA is better with 100% mileage accural on all Y fares, but TG is more limited even with the Star Alliance connection. Some discounted Y fares can only qualify for 50% accural or no mileage at all.
Carfield