Given that need-for-speed-most-wanted is yet another game name which blows past the 25 character tag limit, I'd say we should encourage the nfs abbreviation. It's a well known abbreviation for the series anyway, and we should be able to make synonyms for the full name versions.
It is consistent with other tags already used - or at least, with many tags I know of.
The most important part of a tag is its beginning, because of auto-complete. So I really want a need-for-speed game to start with need-for-speed. And for users who are choosing those tags the otherwise cryptic "hp" or "mw" abbreviations become obvious, since they are limited to the need-for-speed domain.
The NFS series is relatively well-known and probably has very few other games competing with the initials, but other gaming abbreviations may not be unique, so I think it's best to start with the full name.
I admit, though, that many nfs players are very likely to use the initials as well, so it might be wise to set up synonyms.