veganbikes

What makes a bike good is quality and bikes in the lower price points lack the quality. For a fixed gear bike you want nice round chainrings and cogs made of stiffer materials, typically that comes from precise machining and higher quality aluminum or steel. The cheaper stuff is stamped out of cheap materials and that can lead to less precise drivetrains so you might notice more tight spots. Also in terms of freewheels they use cheap stuff that cannot be maintained and probably will use lower quality bearings and again very poorly machined or just stamped parts. All of this leads to a very noisy, clunky drivetrain instead of a smooth quiet drivetrain.

Also bearings are a big thing and generally with cheaper bikes they will use lower grade ball bearings in a cage rather than sealed cartridge bearings especially in places like headsets and wheels. However it should be noted for track racing you will find loose ball stuff frequently but since that is designed for track racing the bike will see very little rain and other commuting stressors and they will likely run thin grease or oil to go faster but they are using very good quality ball bearings and races and have professional mechanics to maintain their stuff or they know how to do it themselves.

One big part of the bike that uses those bearings would be wheels which on cheaper bikes will mean machine built wheels made with cheap parts and not touched after building so they can frequently be out of true or out of round or more likely to break spokes. Braking surfaces can be poorly made and the finish can be rough meaning potentially more chance for flats. A decent wheel is going to be at least finished by hand and a really good wheel is going to be handbuilt by a good wheel builder and of course will use high quality parts. Some really cheap bikes use heavy large spoked wheels usually made of metal or cheap plastics to attempt to mimic the carbon fiber wheels a pro cyclist might use. Some also use very deep section wheels which look cool and certainly can be aerodynamic and potentially stronger (due to shorter spokes) but are usually quite heavy and again poorly made.

In terms of frame materials cheaper bikes tend to use stuff like hi-tensile steel which tends to be heavy and have a less lively ride then higher quality steels that start around 4130 chromoly and get nicer. They might also use cheap aluminum alloys which can give a rather poor ride quality and again be heavy. Frequently as well they will use a lot more stamped parts and poor welding leading to a potentially less durable frame. Sometimes they will also use 1" head tubes which were common on vintage bikes but have lost more favor with 1 1/8 or tapered stuff being the standard now. 1" isn't wholly bad because you can still find quality threaded headsets from Chris King, Cane Creek, Campagnolo, Hatta and Tange and some threadless stuff from Phil Wood and some of the makers mentioned earlier but stems can be harder to find so you might need to shim out or have stuff more designed for smaller diameter bars, same thing with forks, it is much harder to find a lot of quality forks in 1. They might also use odd sizes of seat posts so instead of the relatively standard 27.2 you might get something smaller and have a harder time finding a quality seatpost should you need it.

Also one big thing with lower cost bikes is machining tolerances and quality so you might get something where things don't thread in well or aren't properly faced and chased so things can be uneven. They also might use parts of cheaper materials which may not handle your riding well.

A cheap bike is OK enough for a lighter rider who is riding the bike very very very occasionally but if you are putting in miles it may not hold up well. Save some money and buy a higher quality bike and it will pay off in lower maintenance and less costs in the long term replacing parts and such. Also when you buy a bike from a bike shop you will get an already built bike that should have been checked over for any issues from the get go and probably once more when you purchase the bike wheras the cheap stuff found online it is poorly put together generally by people who aren't mechanics and don't know what is the correct way to assemble a bike so things can be too loose or too tight or improperly adjusted and they pass on the cost of getting it properly built to you.

One big thing I always recommend is actually test riding the bike and getting a feel for it which is not really possible to do with these online sellers. Getting a feel for the bike beforehand gives you a bit more knowledge coming into your decision plus you can talk with someone who should be quite knowledgeable about bikes and can help guide you. Plus when you buy the bike through that shop they can help you with warranties and maintenance down the line which can be quite handy. Trying to go at warranty alone with no support is tough because a lot of bike manufacturers don't want you doing that and expect you to go through a shop and a shop that doesn't sell that product may also have a tough time getting support.

If you are looking for something not too expensive but of decent quality look for stuff that might have Andel or Sugino cranks, sealed cartridge bearings, 1 1/8 steerer tube, 4130 chromoly or something from Reynolds, Columbus or Tange (which might be a variant of 4130) and other features that I had talked about in this thread.