I agree it shouldn't be too restrictive, but you should be somewhat capable in at least some of the appropriate skills
This! This is exactly how I feel!
There's a line. Guilds and factions should, for the most part, operate in real life. If you join... fine... but you'll never rise above your level of incompetence. You'll hit a wall where, unless you suddenly learn more and become more skilled, or dig deep and find some natural talent you didn't know you had... you just can't breach.
I'm all for making alternate methods, mind you.
If your Warrior wants to join whatever the Mage-Faction is... let him learn a couple of parlour tricks and then try to schmooze his way up the ladder. Bribery, flattery, and sleeping with the boss are all widely acknowledged (albeit shady and deeply frowned upon) methods of gaining status and prestige and that promotion you want. Is it advisable, not really... because you might just find that you went and got yourself wrapped up with someone who's only going to use your few talents to their own ends and cut you loose... but that is the price you pay.
If they did it that way, I would totally love it. That'd be fine.
Skill requirements, with a few outside ways of getting around them... if you've got some other talent for doing so.
But not having skill requirements is a deadly, deadly mistake... and it was probably the single-largest flaw with the Oblivion factions they made. If they had just had that, I might very well have found the faction quests to be more interesting. Instead, they didn't really even -require- any unique skillset. Why were all the Mages Guild quests entirely without dependence on magical skill? Why were the Dark Brotherhood missions all so similar to Thieves Guild missions, with the exception of setting someone up to die?
They should be unique, and specialized...