Different sensibilities, I guess. I can't really explain it well because it's a position I don't really understand- I feel that magic users being able to use magic in place of mundane activities adds to the class fantasy- a Wizard shouldn't have to carry around a crossbow or a flint and steel or a torch- he has magic for that!
However, here's a few common complaints that might shed light on things:
1- infinite, hands-free source of light. In old school play, carrying around torches and lanterns has an opportunity cost. You need to have a free hand occupied, and you run the risk of running out of light in a dangerous place if you don't carry enough torches and/or fuel. Magical light used to take a spell slot, so again, opportunity cost. If you're running an exploration-heavy game, having every caster able to provide infinite light sources is a major paradigm shift.
I'd just point out that there ARE good answers for this. So, for instance Dungeon World spell casting always involves risk. The Wizard has a light cantrip which acts like a pretty-much-ever-burning torch, BUT you have to cast it! On a 7-9 there's a consequence. Now, with a +3 INT you will get 10+ on a 7, so it isn't THAT dangerous, and the 'forget this spell until you rememorize' is often going to be no consequence at all, but in that game the GM is always free to say "well, hard move, I take away your light!" so its not a 100% reliable source of light. 6- isn't specified for spell casting, so that's also GM hard move territory.
2- in a similar vein, if you want ranged attacks, you have to carry around the weapons to make them. Range penalties and ammunition are issues to consider as well. Along comes Mr. Sorcerer with his infinite fire bolts, which deal elemental damage (often superior to regular damage) that also scale as they level up to being better than weapon damage! Not to mention the obvious advantages of always having fire on tap. An old school encounter with a troll, where a character might have to give up their normal attack to hit a troll with a torch becomes a much more simple affair as the caster lights it on fire every turn. And there's always that one guy who complains if you can keep hitting a normally difficult to destroy obstacle with limitless damage with the DM unable to say things like "keep doing that and your weapon might break or you'll get tired from swinging it", etc..
Again, this has been pretty much dealt with by DW in the same way. In 4e at-wills are OK, and you can get by with them most of the time, but if you want a large effect, you will need to cast encounter or daily spells, or even delve into rituals. Sure, you can always have Fire Bolt on hand, but then you don't have Cloud of Daggers, or one of the other useful at-wills. Everything is a trade off...
3- the classic balance point of casters vs. martials is that martials can do their thing all day long, and casters run out of magic, so it's a powerful resource that must be conserved. At-will cantrips massively disrupt this paradigm.
That's just a balance that EGG/DA struck in their early play. It isn't even the only paradigm in classic D&D, let alone modern D&D.
How you approach the game fundamentally shifts, and if your gaming experience consists of slowly creeping through dark tunnels, wondering how far your resources can stretch before you have to flee, at-will magic will probably feel quite jarring.
Yet the same people who complain about 4e powers don't generally complain about the same perpetual light problem in 5e, where it is at least equally acute. Now, 4e indeed makes it easy to get permanent light sources, you can JUST BUY ONE, but again that same item, Sun Rod, was equipment in 3e also. I think the point is, modern D&D just isn't really about old school dungeon crawling. I tend to like the idea of problematic light myself, it can have atmospheric and tactical fun uses, but it isn't like its the big stick in my bag of tricks. 4e is not, nor was 3e, nor is 5e, a good game for running OSR-style DC. Every one of those games inherently assumes PCs can mostly take care of the mundane stuff. I mean, food, 4e has a rudimentary starvation rule which the GM can choose to employ, probably as an SC failure consequence or as a 'stick' to put pressure on the PCs. Its not like 1e where ration use is carefully documented and hunger/thirst kick in at specific points, etc. MAYBE you could play 4e that way, possibly. The rules don't really abolish older-style challenge play of that sort, but it probably won't be the most fun way to do it in 4e. 5e is sadly lost here, it lacks both the framework to do it old-school, but nor does it have 4e's more story-driven infrastructure with things like SCs. Effectively in 5e starvation is just an annoyance that the GM could impose on the players!