This is something that even WotC had trouble figuring out back in the day:
What is 1 character level (or 1 hit die) worth?
In its simplest form, it's worth at least 1 HD with associated skill points, BAB progression, and save(s) progression. Sometimes, spellcasting (prepared or spontaneous) is progressed. (You can substitute casting here for other similar systems like manifesting, binding, martial maneuvers, and maybe also Incarnum.) Sometimes class features are gained in addition to or instead of casting.
Taking the first level in a class as the first level overall provides 4x skill points and usually max HP for that HD. Taking the first level of any class also grants proficiencies and access to a new list of class skills which may overlap with ones the character already has. If favored classes are in play, other penalties and bonuses apply.
Answering the thread's title question is understandably complicated since taking Fighter1 at level 1 provides a variety of proficiencies, a bonus feat, and likely also max HP for that HD, while taking Fighter1 after Wizard5 is more questionable. (Certain builds intend to do this. I acknowledge that.)
Level adjustments further complicate things since they give features in the short-term while normally giving none of the scaling level-based abilities (HD, skill points, saves, and class features), and when a level adjustment is gained also matters. Being stuck level 1 with a notable level adjustment is painful, and probably more painful overall than having that same level adjustment at level 5 or 10. (I am aware of level adjustment buyoff from Unearthed Arcana.)
Racial HD also complicate things since they generally give less than what an equal number of class levels would give, but at least they can give skills, HP/HD, BAB. and base saves.
Thus, what is 1 character level (or 1 hit die) worth? What standards exist or should exist?