I've read I Am Spock and it occurs to me that basically, as an actor, he was spoilt by his experience on Star Trek. I don't mean that in a derogatory fashion. Just that he got to play one of the greatest characters in all time in that show, a fascinating (no pun intended), rounded character who continues to appeal 40 years later. Spock was as fully-fleshed out as any character you may think of in today's perhaps more sophisticated tv series (your Sopranos, BSGs, Mad Men etc).
Little wonder that playing a character who was little more than a sipher - and to be fair to the writers of MI, most tv characters were at that level in the 1960s - was boring after his Spock years (or ears).