Constitution is everyone’s second-most-important ability
That’s it, that’s all it comes down to. There are elf subraces without a Constitution penalty; those are fine. But for “Elf” race, and most elven subraces, you get −2 Constitution, and that’s just terrible. Even if Dexterity is your most-coveted ability score, you can get +2 Dexterity without taking a −2 penalty to Constitution, and since that is an option, you will want to do so because even if Dexterity is more important, Constitution still is important.
And it is really important. A +2 to Dexterity means you get +1 to some rolls. They can be some important rolls, initiative, attack, stealth, maybe damage if you work at it. But it’s still just +1.
But Constitution, unlike most other ability scores, has a multiplicative effect (Intelligence does too, with skill points, but that only matters to some characters; HP matters to everyone). You don’t lose 1 life with a −2 to Constitution, you lose 1 life per level. And that winds up being a pretty big chunk of life.
Also, and this is admittedly very much secondary, Fortitude saves tend to be (much) more important than Reflex saves. Reflex saves tend to halve some HP damage, and direct-damage, particularly magical direct-damage, as in the sort of thing you can halve with a Reflex save, tend to be kind of mediocre. Fortitude saves often ward off very unpleasant things, like ability damage (disease, poison) or straight-up death. All else being equal, Fortitude tends to be the more important saving throw. Will tends to be even more important (because a failed Will save also often involves death or ability score damage, or worse can mean possession or mind-control).
The rest of the elf racial features are just... minor
The rest of the elf racial features are nice, but niche or minor. They just do not measure up to the sheer loss that the −2 Constitution represents.
The trance thing rather than sleeping is cool, but ultimately it doesn’t represent a dramatic advantage; rotating guards are still a really good idea, as are alarm spells and the like. Outside of very-low levels, sleep is not a major danger. Note that trancing does not reduce the amount of sleep that arcane spellcasters need. If it did, that would be a really big deal (possibly too good but really, I doubt it), but it doesn’t.
Automatic searching is cool, except if it’s really important you’re probably already searching for it (so the racial ability often just winds up being a minor thing the DM threw in to give the elf player a bone).
In most games, it’s the kind of thing that, if no one was playing an elf, somehow just magically wouldn’t have been an issue in the first place. DM-time is a valuable thing to be spent wisely, which means it’s usually a bad thing for a DM to give much thought to a room the PCs are never going to find. If the players need to find the room and don’t have an elf, the DM is going to find some other way to hint at it, or just move it somewhere that they’ll find it. DMs often do similar things for parties that lack trapfinders or trackers. The lack of these things may, in theory, reduce the party’s effectiveness at a particular task, but the more important the thing to be found is, the more likely the DM is to make sure the party finds it.
In really sandboxy games, the DM might give some thought to random rooms, trying to imagine where those rooms would logically be from the perspective of the building’s designers. Or, if you are playing a pre-made module, it’s entirely possible that the authors of the module threw in random hidden rooms just because someone might be playing an elf.
In other words, this feature can, and possibly will, occasionally get you a nice little thing. It might provide a short-cut, an extra bit of treasure, whatever. Those are nice things. However, even when there is something to find with auto-search, the room still cannot be crucial if it’s unlikely to be found by parties without elves (or the party elf just doesn’t have a lot of ranks in Search or just flubs the Search check). Because if the room is crucial and the party is unlikely to find it, there’s a high risk of the game suddenly grinding to a halt while party has no idea what it is they need to progress. And no matter how you slice it, that’s pretty bad for the game. So this feature will just about never make-or-break the campaign, or a character’s success, unless the DM very artificially forced something in just to make the feature important (see This Looks Like a Job for Aquaman).
Weapon proficiencies are cheap and easy to come by for almost everyone who cares. After all, players don’t play commoners. If you actively use weapons as an adventurer, you almost-definitely already have proficiency in the weapons you want to use. Very few classes swing weapons regularly, yet don’t have proficiency in the weapons elves are automatically proficient in. And even when that does happen, they usually have something almost as good (a Simple variant of the same weapon, for example; the difference is a minor +1 to damage, on average, for having the Martial version).
The one sort-of major exception, wizards who cannot cast spells, still isn’t really a great example, for a couple of reasons.
First, between all the methods wizards have to maintain their spell slots, the quantity of spell slots they have available, and their own ability to decide whether or not to put themselves at risk of needing spell slots they don’t have, most cases of this are fairly contrived.
Second, a wizard spending any significant amount of time where he has nothing better to do than shoot a bow he’s crap at shooting is an indication that there is something seriously wrong with your campaign. In essence, a player who has chosen to play a wizard did so in order to cast spells, not to be a piss-poor archer. He is not good at shooting a bow, be it a crossbow or a longbow, and it’s not what he’s here to do. Which means that, if you as a DM are doing your job right, it should be very rare that the wizard is reduced to plinking things with a bow. The wizard should have better things to do be doing the overwhelming majority of the time, because otherwise you’re being fairly rude to the player in question, invalidating his choices and wasting the time he put into building the spellcasting abilities of the character. Having “better things to do” might not be casting spells; throwing characters outside their forté can be interesting if done right. But having the player sit there going “well, I guess I’ll take another pot-shot with my bow; can’t really do anything else,” means you, as a DM, are failing that player for as long as that goes on. If it’s brief, fine: not everyone needs to have the spotlight at all times; that’s impossible. But if this is going on a lot in your campaign, you are doing something very wrong.
Finally, if I as a wizard really care about this, I can always just be Human and take Martial Weapon Proficiency as my bonus feat, or swap Scribe Scroll for it using the Unearthed Arcana variant, or whatever. I don’t get the +2 Dex, but I avoid the −2 Con, which is going to matter to me more and more often. Even if I’m worried about being reduced to plinking away with a bow, I don’t need four proficiencies, just one. I definitely don’t want to get into melee with a d4 HD, a −2 Con, and a rapier, that’s for damn sure.
So, ultimately, those racial features, while kind of nice, just aren’t worth it for −2 Constitution.
There are exceptions, of course, particularly for elf subraces.
Several elf subraces have bonuses to Intelligence, which are otherwise very difficult to come by (though the same argument about Dexterity applies to Intelligence, with a limited set of books you might not be able to get +2 Intelligence without taking −2 Constitution).
There are a select few feats and prestige classes that require proficiency in particular weapons, and are designed for classes that wouldn’t ordinarily have them. Sometimes being an elf can help with that and the −2 Constitution is a price worth paying if it means not wasting a level in an otherwise-worthless dip, or burning multiple feats, on those proficiencies. Though for the life of me, abjurant champion is literally the only case of this that I can think of.
A few elf-only options are quite good, like the Elven Generalist Wizard ACF, or the Eternal Blade prestige class. The existence of elf races without the −2 Con penalty usually means you should pick one of those, however.
So elves sometimes do have things to recommend them, but they are few, and playing an elf is literally a cost to entry. The thing you get might be worth it, but if you could do it as a not-elf you would.
