Spawn rooms are the first thing you see in a map (aside from the spectator views). You might think they're just rooms, but there's lots of ways to do them right, and lots of ways to do them wrong. I've put together several examples, each with pros and cons.
Example
Here we have a very basic spawn room. It's pretty shoddy on purpose, you can probably guess why. Assuming this is an attack/defend or payload spawn, here are the issues: (yes, this is pretty much Amaranth's original spawn)
1. Blue don't have straight access to the resupply locker without going through enemy lines of sight
Typically, resupply lockers are helpful during a spawn camping to stop the enemy spawn camping you. It's not really helpful if you can't really get to it because of what it's meant to be preventing.
2. Red have lines of sight into our spawn. That's no good.
This is pretty self explanatory. You can fix it by adding a slide door, removing the door as a whole or just making the area behind it off limits to red.
3. The spawn positions are disorienting for Blue
This is probably the least notable until you're in-game. It's pretty obvious as soon as Blue spawn, they're going to be running through those 2 doors in a row which puts them out onto a flank. This one really is much more dependant on the map. However 2 main gates and a flank gate is pretty typical for a spawn room, sometimes 2 flanks if necessary.
Naturally, players will be annoyed having to suddenly turn 90 degrees. It's not majorly game breaking, but it definitely upsets the flow; idealistically players would flow out of the main two gates, using the flank to be sneakier or as a plan B against spawn campers.
Example Fixed
It's obviously pretty varying what you'd do to fix the issues with your spawn room, but this is one potential outcome. So what've we done?
I really don't feel like it should be necessary to go so far into everything you can do wrong. It's more about what a spawn door should do right. Let's take a look at some official maps.
1: Badwater - Blue
Badwater's blue spawn is very well designed. Firstly, there's no sights straight into where you spawn. Secondly, there's plenty of route variation. Spawn camping is little to no issue here simply because the spawn exits stretch across the bottom part of the map, not to mention it's easy for anyone to simply go another way to the one the red spawn campers block.
Easily the best thing about the spawn though is the way it flows. Being in the center-ish, players stay open minded to most of the routes. The furthest (left route) being a less traveled flank - because of it's distance - yet still used simply so players aren't cannon-fodder for Demomen and Soldiers.
2: Dustbowl - Stage 1, Blue
On the flip side, Dustbowl's first blue spawn is bad for pretty much lacking everything Badwater's first blue spawn has - which aren't really unique to badwater, they just make a good spawn room - which really puts the map's first stage in the "ugh" zone. I'm not hating Dustbowl just for the hell of it either, this spawn could be majorly improved.
1. Red demomen and soldiers have a pretty mean shot into spawn. At the point of the gates opening, a soldier or demoman can sit there - firing a few explosives - and rack a fair few kills, then maybe explosive jump away.
2. The flow is majorly upset. People ignore the right route because it's so long and takes you to an area fairly easy for snipers.. That's literally just in front of spawn.. For all that walking.
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The middle route puts you at a major height disadvantage, has no cover and not to mention is right in the middle of the area; meaning everyone constantly has their sights on somewhere of this spot. Don't forget it's long and straight too, meaning it's easy play for snipers.
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Lastly, take the left route. It's a major height disadvantage staircase with an open door at the top. When is that a good main spawn exit, Valve?
3. The resupply locker is right in the sights of anyone who decides to camp the easily camped main spawn door. Do you know what else is outside this easily camped door? A full ammo pack, and beside it a FULL HEALTH PACK.. What the crap did the blue team do to you, Valve!?
I'm easily pretty mad now, so I'm going to end this here. Dustbowl better not still be here when I come back.
Example

Here we have a very basic spawn room. It's pretty shoddy on purpose, you can probably guess why. Assuming this is an attack/defend or payload spawn, here are the issues: (yes, this is pretty much Amaranth's original spawn)
1. Blue don't have straight access to the resupply locker without going through enemy lines of sight

Typically, resupply lockers are helpful during a spawn camping to stop the enemy spawn camping you. It's not really helpful if you can't really get to it because of what it's meant to be preventing.
2. Red have lines of sight into our spawn. That's no good.

This is pretty self explanatory. You can fix it by adding a slide door, removing the door as a whole or just making the area behind it off limits to red.
3. The spawn positions are disorienting for Blue

This is probably the least notable until you're in-game. It's pretty obvious as soon as Blue spawn, they're going to be running through those 2 doors in a row which puts them out onto a flank. This one really is much more dependant on the map. However 2 main gates and a flank gate is pretty typical for a spawn room, sometimes 2 flanks if necessary.
Naturally, players will be annoyed having to suddenly turn 90 degrees. It's not majorly game breaking, but it definitely upsets the flow; idealistically players would flow out of the main two gates, using the flank to be sneakier or as a plan B against spawn campers.
Example Fixed

It's obviously pretty varying what you'd do to fix the issues with your spawn room, but this is one potential outcome. So what've we done?
- Removed/reduce the sights into spawn
- Fixed a disorienting blue spawn
- Re-placed the resupply locker
I really don't feel like it should be necessary to go so far into everything you can do wrong. It's more about what a spawn door should do right. Let's take a look at some official maps.
1: Badwater - Blue

Badwater's blue spawn is very well designed. Firstly, there's no sights straight into where you spawn. Secondly, there's plenty of route variation. Spawn camping is little to no issue here simply because the spawn exits stretch across the bottom part of the map, not to mention it's easy for anyone to simply go another way to the one the red spawn campers block.
Easily the best thing about the spawn though is the way it flows. Being in the center-ish, players stay open minded to most of the routes. The furthest (left route) being a less traveled flank - because of it's distance - yet still used simply so players aren't cannon-fodder for Demomen and Soldiers.
2: Dustbowl - Stage 1, Blue

On the flip side, Dustbowl's first blue spawn is bad for pretty much lacking everything Badwater's first blue spawn has - which aren't really unique to badwater, they just make a good spawn room - which really puts the map's first stage in the "ugh" zone. I'm not hating Dustbowl just for the hell of it either, this spawn could be majorly improved.
1. Red demomen and soldiers have a pretty mean shot into spawn. At the point of the gates opening, a soldier or demoman can sit there - firing a few explosives - and rack a fair few kills, then maybe explosive jump away.
2. The flow is majorly upset. People ignore the right route because it's so long and takes you to an area fairly easy for snipers.. That's literally just in front of spawn.. For all that walking.
-
The middle route puts you at a major height disadvantage, has no cover and not to mention is right in the middle of the area; meaning everyone constantly has their sights on somewhere of this spot. Don't forget it's long and straight too, meaning it's easy play for snipers.
-
Lastly, take the left route. It's a major height disadvantage staircase with an open door at the top. When is that a good main spawn exit, Valve?
3. The resupply locker is right in the sights of anyone who decides to camp the easily camped main spawn door. Do you know what else is outside this easily camped door? A full ammo pack, and beside it a FULL HEALTH PACK.. What the crap did the blue team do to you, Valve!?
I'm easily pretty mad now, so I'm going to end this here. Dustbowl better not still be here when I come back.