3. Monophonic vs Polyphonic
When you come from the VST world, it's easy to forget that there was once a time when synthesizers could only play one note at a time, due to the limitations of the oscillators. That time is long past, but Monophonic, Duophonic, and Polyphonic synths are all still around, still being made, and will likely be around forever. So how to choose?
First and foremost, a monophonic synth can only play one note at a time. No chords, or lush harmonic strings or pads will be (easily) gotten from a mono synth. They tend to be analog synths, and so thick basslines and distorted leads are what mono synths are most commonly used for.
A duophonic (sometimes called paraphonic) synth can only play two notes at a time. They're not super common, but they pop up occasionally.
A polyphonic synth can, as its name denotes, play many notes at once. A poly synth will typically range from 4-100+ voices, depending on the complexity of the patch. With a few exceptions, poly synths tend to be digital, and as such can create sounds and timbres no analog synth can generate. You can also write lush choir and string and pad sections, create sounds with long release tails, and layer chords in ways no mono synth is capable. As an added bonus, most poly synths have a Mono feature enabled, allowing you to turn the poly synth into a mono synth for specific sounds.
And due to the generally digital nature of poly synths, patch storage and recall is a feature of all poly synths, allowing you to store and bring back up presets and sounds you made, for future use.
4. Effects
Effects are a somewhat divisive issue in the synth community. Some love them in their synths, others prefer you only play and record synths dry (then use outboard effects).
Again, it largely boils down to personal preference, and the type of gear you're looking at. Analog synths tend to be more minimal on effects. You might get a Distortion effect, maybe a Chorus, and perhaps a Delay effect, if it's slightly newer. Moogs are famously minimalist in this department. Digital synths usually come with a full battery of effects ranging from Distortion, to Delay, Reverb, Flanger, Chorus, Phaser, possibly Saturation, and multiple types of distortion. Digital synths with lots of effects tend to be more expensive, but in most cases, the effects tend to be quite good (the Virus TI2 has some of the best onboard effects on the market).
When considering buying hardware, considering the onboard effects as well as the filters and oscillators will also be worth investigating.
5. Finally...
Do your homework. There are hundreds of synths out there, each with their own special timbral characteristics and features, and it would be impossible to try and list them all here.
As a general rule of thumb though, based on genres, these are features you'll need to look for to find a good synth you'll get lots of use out of:
Trance: Polyphonic, with 5-100 voices. Some kind of Unisono Spread/ SuperSaw detune feature to create wide, complex trance synths. Good onboard reverb and delay are nice but not necessary. A good Chorus effect will serve you well. The more modulation options, the better.
House: Depending on the subgenre of house, a Monophonic or Duo/Paraphonic synth will serve you well, although a Polyphonic synth for bigroom house producers will do you a lot of good. Fat Analog warmth for basslines and leads is helpful, as is decent distortion capabilities. Analog filters will definitely be a plus.
Dubstep/ Riddim: Polyphonic Digital, for sounds requiring wavetables and lots of modulation, as well as high enough voicing counts to handle complex wavetable modulation. Onboard reverb/ delay not quite as essential, but solid distortion is a must. The more modulation options the better.
Drum & Bass: Depending on the style of DnB, a good Mono or Para synth will serve you well for Reese basses, deep sub basslines, and intense leads. Either Analog or Digital will work equally well, although the added warmth from Analog distortion might add a nice touch to the basslines.
Techno: Largely depends on style of Techno, but an Analog Mono or Duophonic synth will sound just right for techno. All the warm fat low end and oddball lead sounds will be done quite nicely by an Analog Mono. A Digital Poly or Paraphonic synth will also do super nicely, but the extra voicing and tons of added features are a bit on the overkill side.
With this guide in hand, I hope you can go forward and make a better, more informed hardware purchase.
Play on!