I got to thinking about the root question again last night: How much gun is too much?
There is a bunch of ways to answer this. It's going to be dependent on a lot of things, and there are several inter-related factors. Let me give you an example of several types of too much.
TOO MUCH RECOIL: This is the most common excess. A deer rifle should not be painful to shoot. You should be able to take the recoil from the bench unaided. If you feel the need for a Lead Sled, you're dealing with too much gun. The effect of too much recoil is twofold. First off you develop a flinch, and that can be hard to lose down the road. Second, you shy away from the rifle and only take it out of the closet once in a while. The world is full of big magnums gathering dust and surface rust, because hunters just don't want to shoot them. A good deer rifle should be fun to shoot. Even without a flinch, if you find that you have to prepare yourself to take the shot, you're probably talking about too much recoil. In this case, if you find that a 30-06 is no fun, there are a lot of other options that will still take deer at a distance. I mentioned the 300 SAV as a good choice, but it is not the only one. If you look at a recoil chart, 30-06 is a good dividing line. The '06 was developed in the late 19th century for the average American infantryman to shoot all day, every day. If you find that is too much, you are probably recoil adverse and should not venture too far into deeper water.
TOO MUCH WEIGHT: A good deer rifle should be able to match your needs for offhand shooting. My buddy, SuperCore, found this out the hard way last season. He decided to spring for a gorgeous 300 WIN MAG. However, it was a 9.5 lb rifle. He went to take a shot at a buck at 300 yards and could not hold it steady. It was his one and only shot last season. He ended up with tag soup. On the other hand, my son, Angus loves shooting his mother's 15 lb bench rifle in 30-06. I added extra lead to the stock it so she'd have reduced recoil. It is fun to shoot from a good rest. However, it is a real pain to carry to a deer blind. It is waaay to much rifle for him in the general sense, but it does him just fine when it's just the two of us out for Yute Season in the blind and he can make a fully supported shot. What are your needs for offhand shooting? Mine are pretty minimal. Most of my hunting venues offer some type of steady rest. If I can keep it on a pie-plate at 75 yards offhand, I'm happy. However, if you get some long-barreled long action rifle that you can't keep still, you're going to have problems. A good deer rifle should be able to be shot offhand.
TOO MUCH MUZZLE BLAST: You'll find out what I mean if you ever put a muzzle break on a deer rifle. A lot of guys do this to compensate for too much recoil. Yikes! I tried 44 Magnum pistol for deer a few years ago and after a couple of trips to the range, I found I was flinching even with muffs and plugs. I gave up that project in a hurry. However, you cause it, too much blast will screw you up as much as too much recoil.
TOO MUCH STOCK: Well, it's a combination of things, not just stock length. If the comb is too high, it'll knock you on the cheek. If the stock is too long or short, you won't get a good fit and it'll knock you around. Too much drop? Too much recoil. Bottom line: a well fitting stock is important. Once you have proper fit, you can handle a lot more recoil. I went 25 years with recoil pads on a lot of my rifles, and took most of them off just a few years ago. The pads were not on their for recoil; they were on to extend the length of the stock. My body had changed over 30 years, and my clothing had changed, and it all came together that my length of pull was about 1/2 an inch shorter than when I first got into the game. Once I got that rectified, some of my rifles became much easier to shoot.