What are the most important factors that need to be kept in mind while purchasing a manual flash gun?
To me, a "manual flash" means a simple non-TTL flash. When shopping for those, the five essential things to look at:
There are some other optional features that may or may not be important for you:
By saying "manual flash gun" I assume you mean a speedlite kind of electronic flash with manual controls. Modern flash units pack a lot of goodies and provide excellent chance for customizing your lighting setup. I am familiar with the Canon lineup so the examples below are of Canon flashes but they apply to other brands as well.
The first factor to consider, would be the flash's power (measured in Guide Numbers). For a hobbyist use, most of the time you don't need the most powerful units. This means that, depending on your typical shooting conditions, you don't necessarily need a 580EXII but can easily do with 430EXII (I do not mention the other, cheaper ones, b/c you asked about a manual unit). Anyway, the more power levels you can set in your flash, the more flexible you will be with setting up your shoots. Going from full to 1/128 can be useful sometimes but most of the time you don't go that low in the output settings. But, have at least 4 steps of settable power, which make a difference between full lighting of the subject and some strong fill-lighting. For more delicate work you will want finer control.
Then come the other set of features. Manual flashes tend to have a zoom setting as well. This will let you control the spread of the beam. It is one thing lighting one person for a portrait work and a different thing lighting a group. Some units also offer additional diffuser to spread the beam even further at the min zoom settings.
If you shoot a lot of on-camera Speedlite photos then E-TTL (I-TTL for Nikon) is invaluable. It means that the flash exposure is determined by the exposure metering of the camera and you can shoot automatically.
Tilt and swivel are almost always very handy as it lets you avoid direct flash lighting of your subject. This will let you aim the flash at the ceiling/wall to have a relatively huge light source, casting very soft and flattering light.
For some types of photography, like daylight and action shoots the High Sync Speed feature is important. It will let you fire the flash when the shutter speed is faster than the Sync Speed (usually 1/200-1/250 sec). This is crucial for freezing fast moving subjects.
If you are doing off-camera lighting then you can fire the flash with a cable or some (optical or radio) slave. Canon and Nikon have integrated wireless control where your built-in flash or an external transmitter/speedlite serves as the commander to control the setup and firing of the slave unit. A combination of wireless and TTL is awesome. If your camera doesn't have this functionality then virtually any flash/camera can be paired with a cable but there is no TTL in this case. Radio control will let you put the flash in a softbox, where there is no Line of Sight for optical slaves.
Another factor is the weight. Pro units become very hefty. If you own a small camera then a pro unit will make the combination look pretty weird, and more important will introduce some imbalance in the whole setup. My Canon S5-IS P&S equipped with my 580EXII flash really look like a flash with a camera mounted at the bottom... For extended shooting sessions you will feel the hand fatigue holding and balancing the big load.
Lower in importance are factors like integrated bounce card (easily added by a rubber band and a piece of white paper). External power connector may be important if you shoot very fast or for extended sessions. AFAIK, only pro units offer these power packs.
Syl Arena's Speedliting.com has some good information on the techniques and equipment for flash photography (Canon-centric, though, but applies for other brands as well).
A few quick thoughts
It's first on the list because if you can't afford it, well: you can't afford it ;)
How much light does it put out?
It's worth hitting the reviews for this one, but in this day and age, your strobe should trigger every single time without fail.
If you're shooting slowly this won't be so important.
I can't think of the right term here (edit, someone? anyone? Bueller?), but how many stops can you turn the power down? Also, does it turn down in full stops? half stops? thirds of stops?
How do you get it to flash?
Will it fall apart the first time you drop it a couple of feet?
weight / size
You're presumably plannig to hump this around.
Does it have a way to easily attach colour correction gels?