what would everyone think of a telepath background that gives the 'language' telepathy 30ft
Background? Ehhhh ...
For a class or race, definitely. Maybe even a feat.
[MENTION=1288]Mouseferatu[/MENTION] got where I was going... some monsters have it listed as a language already and don't get anything for it...
and 1st level warlocks can get this:
"Don't get anything for it"...except unhindered communication with any/all creatures that speak a language, you mean? You can just handwave that away for any/everyone as "everyone can understand everyone else" for ease of play. I know there are monsters that have it. It's a nice convenient handwavy way to say, "this monster can speak to anyone [and anyone can speak to it], without having to spell it out. Like "psionic magic" was a nice handwavy way for them to include psionic monsters in the MM. But no one who wants to actually play/use psionics in a game wants to see "Psionic Magic" as the implementation for a class.
Now, granted, I had not thought about the Warlock power...and again, that is hardly nothing...unless one's game involves only combat-after-combat. Then, no, you can't use your ["language"] telepathy to attack others or bolsters your allies' minds. But even then, there's no surprise as to what orders the commanders are shouting at the monster soldiers or never a question when you demand they surrender or they demand you do...when the monster/demon/far realmsian "thing" is delivering their evil scheme monologue...when you want to scout/spy on something you have never encountered before/language you don't know.
For people that like psychics and would want to play psychic characters, "telepathy" needs to do [and is] a lot more than "you can talk to anything."
It needs to involve communication, yes of course. But also, "reading minds" [its most basic use]. It needs to involve knowing when there are sentient minds around. It needs to involve the possibility for paralyzing [or other afflictions] by "turning off" certain parts of the mind, creating illusions within a mind, making one invisible [blocking out the perception of whatever is being made invisible in the target mind], break charms and shield their allies' minds from influence...and of course, all of the obvious enchantment spells effects (charms, compulsions, sleep, mind control, etc...) and all the fun psychic bolt/blast/mind-scrambling/damage attacks on a target mind.
So, for people who want that and see that as what "telepathy" is, again, this background/feat idea as just "giving away" telepathy for chats will not do. Just say, in your games, the players understand whatever they encounter/hear and, maybe even creatures that don't really speak but have a given intelligence[?], understand whatever they say.