Thai are, on the whole, not a bad airline. Their on-board service is usually top quality. Of late, they have been having product problems, particularly in premium cabins. About 45% of their long-haul fleet has been reconfigured, giving biz pax new seats and 50-inch pitch (10 inches more than original product), and Royal First Class changed from 62-inch pitch to a comfy 80-inch pitch with fully-flat seats. You just cannot guarantee which product you'll be on, and for business travellers, they need to know they'll have maximum comfort for their top fare tickets!
Problem: BA have just introduced their new 4-class product on select flights (BA019/020 at present), which runs circles around Thai's products. Other SE Asian airlines are very rapidly implementing new products (Cathay and Singapore for example) which will leave Thai as a second-rate carrier in terms of product at the front end. Cathay even rumoured to be looking at premium Y-cls product like BA's World Traveller Plus.
Friends who live in BKK believe that any airline which is 93% owned by the finance ministry is in deeeeeeeeep trouble... particularly with some of the graft that occurs in Thailand. Too many "ministers" and "friends" will be dipping their hands into the coffers, and leaving the airline to rot. It is already happening (anyone need cheap champagne in BKK? It has "somehow" been made available on the streets...at 50% of normal prices....ooops!) The new president of Thai has ZERO airline experience, except for being a passenger from time to time. Go figure.
Consensus from BKK expats: Okay as a domestic and regional carrier, good at long-haul economy class, but not worth it for front end business as other airline products are much better with similar (or better) service.